Is there a way to prove that someone is using some sort of engine for moves?

any helpful advice?

Run a game through an engine such as Houdini or Fritz.  Or, you could infinite analysis and if the engine recommends the same moves then you have your answer.  If you see no advantage slip (e.g., going from white has a clear advantage to white has a moderate advantage) then that's a tip-off.

 If they make their moves too fast then that's another red flag.  They'd likely be flagged by the website's task-switching detector anyway. 

It's discussed at length here:

http://www.chess.com/groups/home/cheating-forum

ok, and if someone is using that what can you do?

Report them to staff.

http://support.chess.com/Knowledgebase/Article/View/124/0/cheaters--cheating-what-you-need-to-know

I think the staff can check with statistical software if any games were played that don't look very ''human.'' I think it's not unlike checking yourself what your own engine would suggest for your opponent. (But don't do that during the game of course)

ok. thanks very much! :)