There are multiple ways, usually incredible shots and awareness, which is a bit too unnatural. I don't mean when someone is just good, no, i mean instances when someone turns towards you in an instant, usually when entering the room, even if you are hooked to the ceiling and he hits you dead-on all the time. Normal players always check around, they don't know in advance where someone is hiding.
It takes experience to detect skilled cheaters when playing them. There are other ways too, like spectating, slow-motion demos, talking with that person, pretending to be normal player and asking them for help with cheats, pretending to understand their point of view (hehe), etc... -- sooner or later they reveal themselves, because it's statistically impossible to lie successfully all the time.