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?