Children are solipsistic creatures. They tend to think of themselves as the only "real" beings in the world - any parent will tell you that a child learning to speak is followed by a phase where they constantly make demands of the people around them and get furious with confusion if they are not immediately abided. An important (and often skipped) step in becoming an adult thus is realizing that other people are other people, with their own thoughts and feelings independent of yours.
For many, this comes through a realization, often profoundly unsettling, that the people around them can change. Surely you've noticed this at some point in your life - you meet with an old friend after a long absence and find that you barely know them anymore. Or perhaps someone important to you - a parent, sibling, coworker, friend, spouse, child, starts acting strangely, out-of-character. Maybe they're having a very bad, or very good day, and they'll go back to their old selves soon. Or maybe they won't, and whether this new demeanor of theirs brings you closer to them or pushes you back, you know at least that they've gotten a bit more in touch with their true selves, that this change was their own decision.
But what about when it isn't?
There is a way you can test the people that you know. If the test fails, it means the person is just acting unusual, and you can laugh it off and think of the test as a last game to play before you finally grow up. But if it succeeds...well, we'll get to that.
What you must do first is buy a vase of flowers and put them in a place where you and the person share a lot of time - the cubicles you both work at, your dining room, whatever. These can be any kind of bouquet, but they must include baby's breath - just baby's breath would work, actually. As the days go by sprinkle something in the water that will kill the flowers, like salt or weed killer, a little at a time. You can do this in sight of your subject or not, it doesn't matter. Continue doing this even after the flowers have completely died. If your subject asks you about the flowers you can abandon the test now, as it is clearly a negative. If you catch them staring at the dead flowers for long periods of time, however, and especially if they throw them away when you aren't looking, you should proceed to the second phase of the test.
The second phase is to get the subject to have dinner with you. The purpose of this phase is to agitate It into revealing Itself, and there are several methods you can use for this. The dinner should ideally be black pudding, blood sausage, or something else with blood as a major ingredient, but if you live in a place where these are unusual or not readily available any iron-rich food will do, such as red meat or legumes. It is vitally important that the subject not prepare any part of this meal themselves, not even pouring any drinks (It may try to soothe Itself by finding ways to "help" even if asked not to, so be wary).
The area you eat in can be prepared with as many of the following as you can manage: Objects made mostly of iron (the older the better, placed near the northernmost wall), something intended to be burned (incense, cigarettes, tinder etc., to the east), sealed bottles of something liquid (south), pressed or dried flowers (anywhere), the bones of any animal (anywhere, but must be hidden). Finally, for practical purposes, you should have a deadly weapon of some sort hidden on you, and there should be no less than one and no more than three other people within shouting distance, again for practical purposes.
As the dinner commences you should try and steer the conversation to your history with your subject, but intentionally get details wrong. The subject not noticing these discrepancies is not a sign of a positive test (people forget things all the time, after all), but them growing more agitated with each mistake is. Other topics of conversation that can further agitate It include: Excavation and digging, pregnancy and new births, machinery, upcoming astronomical phenomenon (meteor showers, eclipses etc.) and anything that strongly invokes the color green. Finally, repeatedly drawing a circle, clockwise, using the ring finger of your left hand has a strong agitating effect on Them.
The sum of all these steps should cause the subject to become increasingly silent and nervous over the course of the dinner, though be sure not to speak of anything that would upset your subject under normal circumstances to avoid a false positive. At extreme levels of agitation They may exhibit any of the following: lapse of broad motor control, sucking on their food without actually chewing it, uncanny and erratic eye movements and expressions, "twirling" things like forks or salt shakers, scratching (Especially near the stomach).
Sudden increases in vocal pitch, and the unexplained appearance of lesions or lacerations on the subject's skin. If these symptoms become too bizarre to explain away, disassociate yourself with the subject as soon as you can, no matter how much the subject seems to recover afterwards - there is nothing else you can do. If you need to be absolutely sure, there is a third phase, though it has its own consequences.
The third phase is simple. Look the subject directly in the eye for several seconds - They will meet your gaze, no matter how much trouble They have controlling the rest of Their body. Ask the subject, "[Subject's first name], why did you kill his daughter?" using that exact phrasing. If you are very fortunate, your subject will blink dumbly at you and you can laugh it off as a badly botched joke, assured your subject is still the person you once knew. It is more likely, however, that the subject will stare at you for several moments, and you will know, unequivocally, when you see It in their eyes.
Don't ask what It is, you'll know what I mean when you see It. When you see It, immediately break eye contact and reach for your weapon, as It will inevitably cause the subject to shriek and attempt to kill you. Fleeing at this point is possible but unwise, as leading the subject on a chase could trigger attacks from Others you may pass, and there are far more of them than you'd be comfortable knowing. This is also why you need witnesses to help you argue self-defense later, but not too many - They usually attack when They see Others attacking, and you can only kill so many. Lastly, it is extremely important that you not look your subject in the eye past this point, even after they die.
Unless, of course, you want to know what it's like to be one of Them?