from never looking in the mirror ?
they are rheal. I have seen 2, fiery red things, I've seen a taller arch demon too about 20feet tall.
you might see depictions of them on shows like Buffy Vampire Slayer or Charmed, I don't think its their real form, things as they imho do not have a form, they're just immensely capable of fitting in. storys go about they are mostly within army bases, with things like area 51 I think those ones are a bite deceiving because it could easily be a hospital operation, but you know what war is its typically an unspeakable bunch of girations.
you might not have seen demons because you're not near an army base or you're not deemed some sort of risk by the security agencies. also, when you go to a club you don't ring up your horse friend or your reptile mate do you? I think in advanced soceities spoken about in greek like a forward backward thing you do hang out with other species filled with the same how to get along day to day stuff. it's well documented we're not alone, but the typical media view is that even as much as a African man and woman on holiday in Europe causes some people to become murderously mad.
children are said to see them sometimes? not sure, they might be called "monsters" and nobody else saw it razmataz. with things like spottings of the "loch ness monster" the believability dwindles as it does the in the fable about the boy who cried wolf or the gruffalo(s), even lord of the rings, troublesome things like Deja Vu mixed with the matrix. sometimes its just not pointworthy to say unless it was heard in the nighttime. Chinese people are reportedly all robots hehe.