Please refer to this page
http://www.chess.com/forum/view/community/chesscom-policy-on-cheating
We provide tools to report cheating to our staff. But having several constant and redundant threads on the topic that circulate with the same questions and comments over and over again isn't helpful, and instead causes people to worry more than they need to. If you want to discuss cheating, please do it in this group
http://www.chess.com/groups/home/cheating-forum
How does chess.com tell if some one is cheating?

There are alot of cheaters out there, and chess.com catches quite a few of them. But my question is, how? In some cases, it would be so clear that you don't even have to look at it to know, i.e. some one with almost no loses and a 3000 rating with no title. But what about all the other players? What if they are using more than one engine? What if they make their own moves every few moves? Do they test to see if they're "computer-like" somehow? Just wondering.