There are shapeshifters who can perfectly replicate a person, and eat their brains to absorb all their memories. These shapeshifters are cunning and predatory, and so humans form extremely close knit societies to try and combat them.

In these communities, nobody walks out alone, all movements are tracked, all doors are sealed at night. All travelers are shunned. They know that any of their family members might be replaced by indistinguishable shapeshifters, so they stay vigilant.

But, occasionally, mistakes happen - some one wanders out alone, and people start to wonder if it's same person who returns.

Let's say that a family member is suspected of being a shapeshifter. They look identical, and they have all the memories of the original. There is no physical means of proving the difference.

However, the shapeshifter can only duplicate memories, and not personality. By nature, the creatures are inhuman with alien psychology. Questions such as "what would this person do?" can trip them up and cause them to give weird answers.

These shapeshifters are intelligent though, and suspicious of tricks. These questions can't be obvious or else they'll try to deceive their way out. If asked "Do you like cats or dogs?", for example, the shapeshifter would then just look back at their hosts memories and reply "oh yes, I've always kept dogs as pets".

But if the shapeshifter isn't consciously aware of the right answer, then he's more likely to trip up. Thus the questions have to be something about the original that even the original wasn't aware of.

So... How would the community best try and interrogate a person suspected of being a shapeshifter? Are there any pyschological tests or profiling methods that they could use to identify a potential body double with all of the original memories?

And, follow-up question, could an interrogation ever prove the negative - could they ever say with certainty that that person is definitely NOT a shapeshifter? Or would the lingering doubt hang over their heads forever?

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    $\begingroup$ What are the shapeshifting limits of your creature? Can they only shift into a naked human form, or can they also imitate clothing, metal, etc? $\endgroup$ – Kyyshak Jan 20 at 17:12
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    $\begingroup$ "You're in a desert, walking along in the sand, when all of a sudden you look down. You look down and see a tortoise. It's crawling toward you. You reach down and you flip the tortoise over on its back. The tortoise lays on its back, its belly baking in the hot sun, beating its legs trying to turn itself over, but it can't. Not without your help. But you're not helping..." $\endgroup$ – Darrel Hoffman Jan 20 at 21:38
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    $\begingroup$ I challenge the idea that having memories is enough to simulate human behaviour - especially for an alien- so that people don't almost immediately notice that something is *way" off. $\endgroup$ – Gnudiff Jan 21 at 7:14
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    $\begingroup$ What is the technology level of the world? Is it close to real-life? The 90s? Napoleonic times? "Generic medieval"? $\endgroup$ – VLAZ Jan 21 at 7:41
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    $\begingroup$ If they have all the person's memories, isn't it basically the same person? The reason I feel like one person with an identity persisting in time is because I have all my previous memories, not because I do a constant personality test on myself. It seems like if I became a victim of a shapeshifter, from my point of view, I would still be alive after they eat me, but I would just have weird inexplicable personality differences. (As if I hit my head really hard, or something.) Wouldn't the shifters "believe" they're the same person as their victim? (And would they be "wrong" in that assessment?) $\endgroup$ – Bridgeburners Jan 21 at 15:33

18 Answers 18


This is similar to the problem of finding replicants in blade runner, so just apply the voight kampff test. the point is that you ask questions that that are supposed to evoke an emotional empathy response which your alien minded creatures don't have in the same way the humans have so it would be possible to notice.

Another option, that is used today, is gait detection. Copying a persons walk is surprisingly hard and assuming your shapeshifters only have access to the conscious memories of a person they will have no way to know exactly how he walked as that is not something you think about

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    $\begingroup$ So anyone working in security cleared functions has to do a couple of years at the Ministry of Silly Walks before clearance? $\endgroup$ – Separatrix Jan 21 at 15:21
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    $\begingroup$ Interesting addendum, there are studies showing that people can correctly identify family and close friends by their gait at extreme (multiple-kilometer) distance and even by sound alone. I've experienced it myself with my fiance and my flatmates, even when I didn't know what clothing they were wearing my eye is drawn to people I know. My Fiance and I make a habit of guessing which flatmates are walking around at home and generally base it on the sound of their footsteps too $\endgroup$ – Ruadhan Jan 21 at 15:27
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    $\begingroup$ @Ruadhan: More generic example: I could determine who was coming up the stairs by their walk, always. $\endgroup$ – Martijn Jan 22 at 9:16
  • $\begingroup$ The gait idea is awesome. I remember once being startled and alarmed when I saw someone at a distance who 'looked like' a person I was afraid of, only to realize a few seconds later that the person actually bore very little physical resemblance but -walked similarly-. $\endgroup$ – Meg Jan 23 at 18:27
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    $\begingroup$ You had me at Voight Kampff. +1 $\endgroup$ – fgysin reinstate Monica Jan 28 at 12:03

Behavioral Biometrics

In short, muscle memory.

There is a section of biometrics called behavioral biometrics, which is based on your actions being different from person to person, as opposed to simply having a different face or fingerprint. The actions are learned and depend on your physical body, but is otherwise largely muscle memory and not a conscious effort.

For example, the way you type on a keyboard. The millisecond time between key-down and key-up, and between key-up and key-down again, is different enough between people that you can reliably distinguish between them. Even if you were told which key presses were too fast or too slow, it is really hard to change that with conscious effort.

The real person is likely to "behave like themselves" a majority of the time, occasionally being off the normal, while an impostor is likely to only occasionally behave like the target.

It works very well when combined with a password, but can also be done on a continuous basis.


Example implementation: Each citizen have a short passphrase that they use to identify themselves with. The phrase itself is long enough that you get sufficient keystrokes to reliably authenticate the person. If the people don't know about the biometrics aspect of it, they will only think of it as their secret passphrase.

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    $\begingroup$ I think that the best way to interrogate muscle memory would be a field test by persons familiar with the individual. Play sports with the individual or have sex with the individual. Or both; probably not at once but if so this could make for a lively story. Bonus - if you are the real person, no reason to dread the test. $\endgroup$ – Willk Jan 20 at 17:35
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    $\begingroup$ More in the muscle memory direction is dance. If you ask a dancer how to do a certain move they'll give you a blank look, but if you ask them to do it they'll instantly get every move right. Dance moves aren't conscious memories so if you teach your tribe members a dance, the shapeshifters won't be able to reproduce it. $\endgroup$ – Muuski Jan 20 at 18:05
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    $\begingroup$ +1 for the concept, I would think of sports as a more obvious solution to testing muscle memory, many sports are very technical in nature (and not just based on pure physical strength). The ones that come to my mind are martial arts, skiing, mountain biking etc. As pointed out by @Muuski , dancing is another such example, it has the convenience of being present in almost every culture and requiring little to no infrastructure. $\endgroup$ – Alexandre Aubrey Jan 20 at 22:25
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    $\begingroup$ Or just use a good old handwritten signature? Even if you have a sample, it's still difficult to take one, if you only have limited time. $\endgroup$ – infinitezero Jan 21 at 11:29
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    $\begingroup$ Dancing would have the advantage that it can easily be integrated into other rituals without it feeling like an interrogation. Having a dance party when someone returns from a trip where they perform in the traditional welcome-dance routine could be a regular cultural tradition which is first and foremost a social event but would also solve the secondary purpose to identify shape-shifters. $\endgroup$ – Philipp Jan 21 at 12:14


Similar to one comment about microchipping your population.

A lot of cultures have a practice of ritual body-piercing. This isn't something that can be replicated by shape-shifting flesh, or by stealing clothing.

The mimic would have to deliberately produce piercing-holes, steal the jewelry, and then have to install it. To me, it appears that that level of control is too much for a simple mimic. For a bonus, place the piercing somewhere that a human can't reach, like the small of their back.

The mimic then has to duplicate the holes and either (a) bend inhumanly to fit the stolen jewelry or (b) get another mimic to do it for them.

Depending on whether the Mimics can produce holes in their flesh to match their victim's, the tells would be freshly pierced (angry red or swollen) holes and/or an outright lack of jewelry.

Everyone in the community would have ritual jewelry like this, so the guards at the gates need only check that the person still has theirs. As a result, your citizens can be pretty confident about who is and isn't human.

  • $\begingroup$ +1 for thinking outside the box. $\endgroup$ – Tom Jan 22 at 14:14
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    $\begingroup$ I wonder if tattoos also would work, they are also artificial modifications of the body. $\endgroup$ – Daniel Frużyński Jan 22 at 15:46
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    $\begingroup$ Awesome answer! I also love the idea of how their greeting would be different from ours. Like, touching ears jackets instead of shaking hands. $\endgroup$ – Alexander Dmitriev Jan 23 at 6:33
  • $\begingroup$ I would think that the shapeshifter also be able to assume some temporary shape (like having extra long hands with extra joints) to reach their back and produce any open holes in back, install jewelry in it, and then close said hole and undo extra long hands. But maybe I'm just too used DS9-class shapeshifters. $\endgroup$ – Matija Nalis Jan 23 at 17:55
  • $\begingroup$ I agree with @MatijaNalis. By definition, a shapeshifter can bend inhumanly. $\endgroup$ – cowlinator Feb 11 at 4:14

Rapid fire questions

I assume the creatures still need time to recall something. Asking multiple questions very fast with little time to answer would most likely trip them up. Make it anything that the person should know but might need any time to recall for the non-person:

What's your favourite colour? Why did you get married to Alex? Where did you play as a child? Where did you get that scar from? Where is the money I lent you? What soup did you bring to the family reunion 3 years ago?

It should be easy to come up a list of questions per person that would be easy to answer for them but not to somebody who needs to search through the person's memories. Just wait until the person is missing and then compile the questions, so an impostor can't anticipate them.

This is likely going to trip up even normal humans, so you should tolerate some error. If they need time to think about a question or two instead of firing off the answer immediately, that's normal. But if they stop and think about each thing, then it's time stop asking and start axing.

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    $\begingroup$ Yeah.... I am not someone who relives his past a lot, and I would be absolutely incapable of answering a LOT of these questions...... Scars? I worked for too long in kitchens to remember them specifically. Soup from three years ago? I don't remember what I ate this weekend. In this instance, with me specifically, a shapeshifter would be better than I am at these questions -_- $\endgroup$ – Patrice Jan 21 at 14:58
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    $\begingroup$ @Patrice in a culture that regularly has to interrogate their close ones you bet you'd have a lot of activities that revolve around reminiscing. 1. You need to have a good enough recollection. But more importantly 2. Others need to know what to ask you and what you'd answer. Games, songs, activities, and other cultural pastimes will be related to getting to know your friends, relatives, and neighbours better. $\endgroup$ – VLAZ Jan 21 at 15:02
  • $\begingroup$ Yeah.... that's a good point. With these questions being a bit more tracked, I likely would pay more attention. I'm thinking about this with my "no risk of being eaten and shape shifted into" brain :p $\endgroup$ – Patrice Jan 21 at 15:04
  • $\begingroup$ @Patrice at least you can rest with the knowledge that w̶h̶e̶n if a mimic eats you, they'd look through your memories and only then find your kitchen experience. This will show the mimic there are much better ways to prepare and consume their meal and they'd be really angry with themselves for now knowing this sooner. So, you get a post-mortem strike against the mimic. And ultimately, the moral victory over them because you were better than they could ever be. You were...until you were eaten. $\endgroup$ – VLAZ Jan 21 at 15:17
  • $\begingroup$ that was a beautiful thought :p I'm sure my spirit will rest in peace better knowing that. Thank you sir! $\endgroup$ – Patrice Jan 21 at 15:38

Brain Scans

Ask them questions about what their emotional response would be to hypothetical situations while they're in an MRI. e.g. "A person pushes you to the ground -- how do you react?". If the section of their brain associated with long-term memory lights up e.g. They are trying to retrieve a memory of a similar event from their human host to know how to answer the question then you've got a shape-shifter. If the emotional section of their brain lights up as they place themselves in that imaginary situation and imagine how they might react you've got a human.



Or, any other similarly complex game such as Go, or card games.

If it is traditional in your society to be wary of outsiders, it's not implausible for it to be customary to play chess to "check" that your opponent is still the same person. Your shapeshifter may understand the rules of chess, or go or tarot or whatever, and may recall what their "host" did previously, but unless they can recall memories more precisely than a human, they probably wouldn't be able to emulate their playstyle.

I'm a big Magic: the Gathering player, and everyone knows that my favourite colour is blue. But they probably don't know that I am absoltuely unable to play an aggressive deck properly and feel most at home in a control deck with heavy spell manipulation and disruption. I'm pretty sure my closest friends would be able to notice that if someone was impersonating me.

  • $\begingroup$ I like this, but it's not exactly "pass the gate-guards" material. More of a 'tell' for a mimic that somehow got past the guard test. Good idea for a plot-point in a story though. $\endgroup$ – Ruadhan Jan 21 at 15:29
  • $\begingroup$ This is really more of a "hey, this is kind of suspicious" thing than a "this confirms our suspicions" thing. Maybe you wanted to diversify your playing style and so have been studying how a red rush deck works? $\endgroup$ – GreySage Jan 21 at 19:37
  • $\begingroup$ It wouldn't be the most sure-fire way of catching the shapeshifter, but for some reason I was convinced that this was for a more medieval / low tech setting. Not sure where I got that from, but I was thinking of methods that didn't rely on sophisticated tech, and this was what popped into mind first. As @Ruadhan said, it makes for great narrative drama! $\endgroup$ – Whitehot Jan 21 at 21:33
  • $\begingroup$ This also requires that everyone in the society is familiar enough with chess (or whatever game) to have a distinct play style. Most people that don't play much probably won't have any kind of distinct style, except for "bad at chess", which isn't particularly informative. $\endgroup$ – Nuclear Wang Jan 22 at 18:02
  • $\begingroup$ @NuclearWang true, which is why I specified that it would need to be a tradition to play every night for example. $\endgroup$ – Whitehot Jan 23 at 13:30

I feel like the best way to catch people/shapeshifters out are unconscious habits, and ways of feeling/expressing yourself. Little tics of the face, twitches of the hand, and fidgety motions may be related to personality more than memory, so a shapeshifter might struggle to replicate those sorts of things. Abstract things like sense of humour are clearly harder to judge, but even if a shapeshifter remembers the things the human found funny, faking/reconstructing a laugh from memory is really difficult, and laughter is often impulsive, so a shapeshifter wouldn’t have time to race through memories and think “would this human laugh at this?” By the same argument, crying - most people can’t cry on demand, and an absence of tears from a normally weepy person may well be a signal of something being wrong. I recognise that emotional responses might not be the clearest/most empirical evidence of an alien invasion, but they might be the things that they simply cannot replicated convincingly.

Also: do shapeshifters have better memories than their human hosts? Or do they only remember things as well as the humans they consume? There’s this classic sort of thing:

“Oh, do you remember the time when we went to the park and we saw the escaped snake?” “Oh yeah, of course!” scans brain: remembers host human doesn’t like snakes “That was really scary!” “Aha! That never actually happened!”

If a large enough/personally important enough group of people assert a thing that you feel like you should remember, you’re often tempted to go along with it and say you do. Only once you’ve agreed in order to fit in, it’s revealed that it never happened at all. I realise this is a bit ham-fisted, but humans aren’t great at remembering absolutely everything that ever happened to them, and shapeshifters might be just as anxious to fit in and act like they’re part of the community, so might go along with things that the crowd of humans test them with. (Of course, if these humans are trained to be entirely honest whether they remember things or not, the shapeshifters may well capitalise on that memory and behave the same way - so, emotional responses may be the better way to go.)

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    $\begingroup$ “That was really scary!” “Aha! That never actually happened!” Except that's quite a human thing to do. It's called confabulation. It's common in the early stages of senile dementia, when memory is failing. But everyone will do it occasionally. Also many people have false memories, possibly created this way, that they will swear are true. Cult leaders and immoral "therapists" exploit and use these phenomena. $\endgroup$ – nigel222 Jan 22 at 13:01

Have daily jam sessions together

Playing musical instruments and/or singing is a good part muscle memory, something that a shapeshifter probably doesn't instantly get from inspecting a victim's memory. A sudden loss (or rarely, gain) of skill could be a good sign of infiltration. This is the same as Spokio's answer.

But you can take it to the next level by having the community members have daily improvisational jam sessions in rotating company. People learn each other's habits and preferences and notice something is "off" when someone plays differently. Since people are not aware of their own preferences on the same level and don't play fixed pieces of music from memory, this is impossible for a shapeshifter to reproduce successfully with any consistency, even if they are so agile that they can pick up a new instrument and play it with skill.

As an extra security level, community members are not allowed to discuss others' musical performances (or lack of it), so a potential victim does not have memories of what others noticed about their performance.

Interrogation is just playing together and happens every day for every single member old enough to hold an instrument or sing. This is of course assuming that the shapeshifters can't reduce their size and mass to child size. Otherwise it wouldn't take long for the communities to die out...

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    $\begingroup$ This reminds me of the movie "The Majestic", with Jim Carrey. The old man piano player knew Jim wasn't the "lost son" because of the way he played jazz. $\endgroup$ – computercarguy Jan 21 at 21:26
  • $\begingroup$ @computercarguy Yeah, it's like that. I almost mentioned Jazz, but wanted to keep it more generic so it can be applied to different settings. $\endgroup$ – Cyrus Jan 22 at 8:50
  • $\begingroup$ What about the people who can't carry a tune in a bucket? (For example, my mom always said that she was helping the church choir by not participating in it.) $\endgroup$ – Martha Jan 23 at 17:34
  • $\begingroup$ @Martha I suppose it will be a relief if they get replaced by a shapeshifter ;-) There are simple percussive instruments that should work, and even they way people fail to keep the tune can be distinctive enough to confirm their identity. $\endgroup$ – Cyrus Jan 23 at 23:50
  • $\begingroup$ A variant of this method was used in the original Earthdawn roleplaying game - because Horror-marked individuals had no creativity, all people were required to practice a creative, artistic skill suited to them and demonstrate it when they arrived at a settlement to prove they were not Horror-marked. $\endgroup$ – KerrAvon2055 Feb 10 at 12:38

Tell the shapeshifter they have Alzheimer's and ask them to take a memory exam

Make it very clear how well they're expected to perform on the test. I.e, "at this stage, you should only be able to answer half these questions correctly. Anything better will be a miracle!"

The shapeshifter will answer half correctly.

The real article will answer all correctly.


I wish I could remember the title, but I read a short story a few years ago about a society with a similar problem. Aliens could infect humans by replacing their minds with their own, but keeping the human memories. The aliens were very logical and didn't do unnecessary actions. The humans had a secret test to identify them.

They police would find some belongings of the suspected infiltrator, wrap them in bubble wrap, and put the wrapped items in a box. The suspected alien was required to unpack the box and examine each item.

It the suspect popped any of the bubbles, they were human.

For your story, find an action that the aliens won't do, and then give them an opportunity to do it.

  • $\begingroup$ Did the aliens take you memory of the title? :) $\endgroup$ – Brandon_J Jan 23 at 19:08

Tell them a Joke

You say - "Your momma's so fat, you'd need three shapeshifters to imitate her fat ass."

Given that alien psychology is different, and further on the lookout for suspicious people, it wouldn't be able to relax or jibe back at you.

A human on the other hand would laugh despite the very real possibility.


Secret Handshakes.

having a very intricate, well practiced secret handshake with everyone in your community. While a shapeshifter may remember all the moves of the handshake, they will not have it practiced well enough to fluidly complete the handshake and will be easily noticed.

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    $\begingroup$ If the shapeshifter picks up the memories, they'd know that the secret handshake is very important. So, they would likely make it a point to practice before meeting anybody. $\endgroup$ – VLAZ Jan 21 at 22:13

Body Tissue or Blood

Maybe extract tissue or blood from the shapeshifter. This works under the assumption that the shape shifter cannot control the matter that leaves its body hence the blood might turn back into its original form over a period of time. The tissue attached to the body of the shape shifter stays in the new form maybe using the energy it receives from the body but if it is detached from the body it should run out of energy at some point and return to its original stable form like how a chameleon goes back to its original color after it dies.


Perform a psychopath test

This article details how psychopaths differ from normal people in terms of imaging what it's like to be in another person's shoes:

... [A] new study shows that there really is a difference between how psychopathic brains and neurotypical brains process emotions, and it might not be as easy as toggling your sympathy on and off. If psychopaths know they're being asked to step into someone else's shoes, they can do so as easily as anyone else. It's when they're tested on their empathy without realizing it that they start to differentiate themselves.

If you took the test, it would work like this. First, you see an image of a room with an assortment of dots on the walls. In the middle of the room is a person facing one of the walls. The researcher asks you, "How many dots do you see?" If the person in the image can see the same number of dots that you can, it takes about one second to answer that question. But here's where it gets interesting. If the person in the image can't see all of the dots (because some are on the opposite wall, for example), it actually takes non-psychopaths about 100 milliseconds longer to answer. The non-psychopath automatically places themselves in the shoes of the person in the image and imagines how many dots that person can see instead of simply answering how many dots they themselves can see. But for a psychopath, the presence of the person and the direction they're facing makes no difference at all.

There you have it: Normal brains automatically place themselves in the perspective of others, while psychopaths have to actually make an effort to do so.

Your shape shifters should be unable to control the millisecond timing of their responses as they attempt to place themselves in the shoes of the other person. As a bonus your society gets to eliminate all of its psychopaths.



In the proper sense, a Meme is a shared idea.

A small community is full of in-jokes and subtle references to things that people have said or done in the past. A shared story that comes very naturally to people who lived it and grew up with it.

That and a mix of things like rhyming slang (thinking of Cockneys here) and it's quite possible to hold a conversation using phrasing that only bears passing resemblance to english.

If the person you're talking to doesn't have any idea what you're talking about, they're probably not part of your community.

For a similar idea: Austin Powers featured a conversation between Austin and his dad in "English English", which was a blur of cockney and englishisms that requires subtitles for most people but technically was comprehensible if you come from the right background.
The bystanders in the scene were predictably mystified.

Of course, given the mimics can use the memories they've gleaned fast enough to have a conversation, they may well be able to cross-reference memories and infer the connections well enough to pass the test.


All depends on the specifics of your shapeshifter.

So they have a sense of humour? Can they replicate empathy? Are they musical? Can they tell doggerel from poetry? Can they comprehend riddles? The test should be set to match their weakness. What sort of weird answers do they give to what sort of questions...?

Also, what sort of IQ do they have? A simple IQ test might weed them out.

Do they have the same weaknesses as what they are copying? I doubt it. If the original is colour blind, or hard of hearing, or tone deaf, or unable to taste parsley, they could be tested like that. Also, are they right/left handed or ambidextrous?

Alternatively, use something like a fire test or ducking: if it kills them, they are declared human, if they survive they fail the test and are executed. Brutal, but it keeps the shapeshifters out.

  • $\begingroup$ "Also, what sort of IQ do they have? A simple IQ test might weed them out." IQ tests are terribly unreliable when used against humans usually. It's possible that an alien could get a very weird score on an IQ test but it's also possible they won't really measure as "different". $\endgroup$ – VLAZ Jan 21 at 22:12
  • $\begingroup$ There only needs to be one sort of question that they're bad at -- which is the odd one out? can you complete this sequence of drawings? -- and you fill the test with those. $\endgroup$ – David Hambling Jan 22 at 18:13

If the technology level is high enough, a microchip in the brain could download a memory each night. That memory would last a day before it is replaced at night by a new one.
Even if the shapeshifters could steal the chip (the chip could send an alert when disconnected or on death), missing an update would leave them with false memories or without the most recent one, a simple test asking about the most recent fake memories will reveal the infiltrated.

For a less advanced setting, I think the best option would be asking for something that didn't happen, maybe even something that contradicts the original person memories. This can make them nervous , did the person forget it before I ate it's brain?, am I being tested or is this just a confusion?, should I admit I don't remember it or do I play along?

It's reaction will probably differ enough from the original human's behavior to confirm suspicion.



We already have a solution for this in real life.

Handwriting is something unique to each person. It is a product of "muscle memory" not conscious memory. And graphological analysis (also known as "handwriting analysys") is an established field with experts and systems of procedures.

In fact, it's so effective that a person's signature is used as proof of agreement to a written document.

It is a fast and effective test with little subjectivity or uncertainty.

It works in any time frame... it was effectively invented by 1575, and today's machine learning algorithms makes it incredibly accurate.