Click on HELP & SUPPORT at the bottom of the screen and take it from there
how do u ban cheaters

They have a peculiar odor.
Do they smell like orange hand puppets with red hair? I'm pretty sure they don't smell like 3rd-world dictators (at least not from Swaziland)

Andy Clifton also guest starred as himself on an episode of "CHiPs," where Ponch pulls him over for driving a car with his big "head" on.

Well one way, I guess, just like in real life when the "man" is on you, would be someone making an admission against interest. Like a player that openly talks about using engines in games, etc.

shh,we're not telling you....
http://www.chess.com/forum/view/community/chesscom---list-of-caught-cheaters

Wait, did you mean how you can tell if someone is currently cheating, or if someone has been caught cheating?
? How to report them...