So, in my world, humans coexist with aliens, and in some rare cases, humans and aliens have exchange programs and all that stuff. This problem goes both for the humans and the aliens. To an alien from, say, Qualis, all humans look a) Ugly and b) the same. Aliens aren’t trained to pick up the subtle differences between two humans faces, and so to them, all humans from any ethnicity look practically identical. And humans can’t see the difference between two aliens either. So, my question is, how could the aliens and humans tell each other apart?
This is a common problem among us too. It happens, for example, when you have to tell animals apart (i.e.: when you have dogs of the same breed and age), or when you have to tell twins apart. It also happens with cartoon characters - in old Donald and Scrooge McDuck comics, it was hard to tell Louie, Huey and Dewey apart, when they could be told apart at all.
It may be made harder for aliens if they have poor eyesight (i.e.: they only see our silhouette), or if they perceive humans mostly with senses other than sight.
If the humans wish to be nice to the aliens, they may wear accessories that make it easier for the aliens to tell one human from another - for example, if the aliens see mostly through sonar, having any object around that would appear differently to an ultrasound scanner might do. They might tell Alice from Bob because Alice has metal earrings (making a jingling sound close to her head), while Bob usually wears belts with big buckles (thus the jingling comes from the waist).
If the distinction must be visual, then allowing people to wear whatever they like will usually have them dressing differently. The aliens would tell people apart through their clothes, kinda like we tell Louie, Huey and Dewey apart in old cartoons and comics.
Another solution which would be really fun to implement would be hats! I played Team Fortress 2 for a long time, and in the beginning it was usually hard to tell two people apart when they were in the same team playing the same class. When hats were introduced, people kinda started wearing different hats based on their achievements and what games they owned, and it the variety of it made people unique. It was much easier to tell who was who because everyone had a different hat.
The same goes for aliens, of course. They could write their names on their foreheads or wear clothes of different colors. As long as they give themselves features which we are good at picking up, we will be able to tell them apart.
Alien side - they cannot tell faces but other distinguishing characteristics are very apparent to them. For example Amy has an inverted nipple on the left, an accessory spleen and smells like ginger. Bert has 8 cervical vertebra, a prosthetic knee and smells like cat food. Cooper has three testicles, each unusually large, and smells like mince. The names the aliens use among themselves for each of us consist of a long list of an individual's physical idiosyncrasies.
Human side - I propose the aliens want to help us because they are sympathetic to our difficulty. We cannot pronounce their names anyway, so they give themselves nicknames with cultural meaning and then adorn themselves with reminder clues. "Scooby" has a green collar with a tag. "Freddy" has an ascot. "Daphne" has a green scarf.
That is a trick I lifted from a story I heard about an American in Turkey - his name was hard for the Turks but he sort of resembled a popular soccer player. So he started going by that dude's name as a nickname. The Turks saw the resemblance, appreciated the joke, and remembered this American's new Turkish name.
Alien AR smartphones will add name-labels to humans
The aliens are at near-future technogy levels. Their smartphones are equipped with Augumented Reality displays, that will automatically run feature-recognition (i.e. facial recognition for humans) query against cloud-based service(exchange agency servers) when encountering hard-to-recognize species.
All of the technology required already exists to this day, it is just too expensive/big to use on large scale for us so this solution would be feasible for us in near-future, possibly already is for aliens. Its is also very flexible (exchange programs many for different species of aliens).
The same way humans do when dealing with other ethnic groups, by learning the small differences that make a particular human stand out from their peers; height, body shape, hair and eye colour, but mainly "manner" how they speak, if they speak, their idioms, their body language.
If aliens cannot "see" differences in humans, perhaps they could identify us by the smell (dogs do that pretty well), or by the sound of our voices, or even the noises we do while walking or moving our bodies. It may depend on which other senses they have available for that. If they have a sense of smell highly developed, they even may be able to "feel" our anger, hapiness or any other chemical change in our bodies. If they have their hearing sense highly developed, they could identify us by our hearthbeats or the internal organic sounds. They could even detect if we have digestive problems.
1.) Sound. They can't tell us apart physically from one another, however, humans have a wide range of unique voices.
2.) Clothing. Not sure if this would work for you setup, but if humans/aliens wear specially designated clothes, then they could be used to identify individuals. Similar to how, when writing a game of risk, one of your options to draw the map is to make a color-map and index the provinces by color, perhaps the aliens have some special ability to identify unique patterns of colors and "index" known humans by them? Haircuts are another option. I usually recognize people by their clothing style/hair style long before I see their face (this is somewhat error prone, but it is correct more often than not). Even ignoring the obvious things, patterns such as how an individual walks or carries him/herself are also easy and quick identifiers.
3.) Behavior. If an alien knows somebody's name and refers to them by said name, and they respond, it is reasonably easy to determine who the name belongs to by their reaction.
4.) Name tags. Seriously, in big black letters. Since humans apparently only interact with aliens directly in a limited fashion (exchange programs), those aliens/humans in the exchange program can simply wear name-tags.
5.) Scent. Pheromones. Every individual has a somewhat unique scent (and some of us have a VERY... unique... scent, if you know what I mean. The same could apply to aliens -- no two smell alike. Once might smell like roses, and the other might smell like cat crap.
Regardless, unless the problem is biological (as in, it is IMPOSSIBLE to physically identify individuals from the opposite species) then the simplest path for both human and alien exchange students is to learn how to identify individuals of the other species.
I would be interested in seeing what you would do with aliens with radically different senses to humans - Eg an alien that could see and interpret the electrical activity in your brain - similar to a shark. People have certain patterns of brain activity which is somewhat unique, so once they learn your name they'd be good at remembering you. On the other hand it could be an important plot point where one character is mistaken for another by the alien revealing that they are very similarly "wired" brain wise (eg both characters are secretly high functioning psychopaths) while coming from very different ideological backgrounds.
"I'm sorry, your mind looks just like a friend of mines. Please put down the gun."
On the other hand maybe the aliens have a concept of "individuality" that has to do with genetic lineages rather than physical bodily units, as determined by a range of senses including sonar and sight. So the Japanese astronaut leading the mission might get nicknamed "Japan" and the Japanese first officer might be referred to by the aliens by a name that translates as "Japan's Right Hand". The human known as "America's Left Hand" gets really offended by this, but "America's Right Hand" finds it amusing.
Why make this task unnecessarily difficult? If you need to distinguish particular humans from the others, why not simply affix sticky notes with identifying codes to the "foreheads" (the relatively flat, furless upper ventral surfaces) of the ones in which you are interested?
But you may want to reconsider whether you truly need to distinguish one human from another. The mere fact that they nearly all have the same number of heads, limbs, orifices, etc is a strong indication that they all provide the same basic functionality.
Well there is the obvious to use senses other than sight. Or....
Perhaps because of this issue people and aliens start wearing identifying marks, like armbands, or patches on their clothes. Maybe it gets to the point of segregation. Or maybe it goes the other route and becomes badges of honor.
Perhaps the issue is so rampant that the governments move in and barcode everyone at birth.
Perhaps society takes it to themselves to setup teaching courses on learning the subtilties of each alien nation.
Perhaps there is a tech company that comes in and sells name badges so that you always have an identifying avatar digitally displayed behind you. Which could be used to infiltrate alien societies or vice versa with fraudulent name badges becoming a problem.
maybe they could read/smell/count/sense a human's microbiome signature- i.e. the unique variety and number of microorganisms living in human bodies.
The aliens would learn to tell the differences eventually if their brains are similar to ours. We have something calked the fusiform face area (FFA) in our brains. https://en.wikipedia.org/wiki/Fusiform_face_area?wprov=sfla1
The FFA activates when looking at faces and is believed to be responsible for differentiating between them. But it also activates when ‘experts’ look at things they are experts at (bird watchers looking at birds, all this mentioned in link). In one experiment people were turned into experts in recognizing made up objects, the FFA activated at the end of the experiment, not the start.
The aliens would have to be taught the ‘rules’ for differentiating faces but even if they don’t have the equivalent of an FFA, chances are they could learn, become experts. As for the rules they could use, this can be a plot point. I’m told black people largely rely on skin color to tell people apart. Personally I pay less attention to faces than height, weight, hair color when it comes to distinguishing the ever changing employees at work that I don’t directly have to interact with. Someone could give the aliens bad rules, or some such.