How to Detect if Someone Assisted by Rybka

Hi all, I'm a beginner.
Can someone explain to me about how can Chess.com (and all chess site) detect if someone are running chess engines when he plays in this site?
If he's truly an expert and his move accidentally the same as what Rybka/Houdini suggests, will he be banned?

I wonder...

Wrong forum. Try http://www.chess.com/groups/forum/cheating-forum.

FYI, Rybka was the strongest chess engine before the title be canceled by the event organizer. Because it was cloned from Houdini, which is open source.

*If I'm not wrong

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

I think Houdini is much younger than Rybka; Houdini's initial release was only last year, though maybe there were beta versions circulating behind the scenes before that.  Anyway, when the ICGA banned Rybka they said it was because it was based on the two engines Crafty and Fruit.