I don't know how you would report it, but how does one know if the other person uses an engine? I'm asking because I really don't know. Also I really don't take live chess too seriously - tome they're just practice.
how do i report a cheater

he was moving at incredible speed probably taking 6sec to move at the longest and just did not make any tactical mistakes we were play G/15min+10sec increment and thru the chat i talked to him a little in his own language using google translate and he didnt respond to anything (of coursthat could mean nothing but it felt suspicious) and what else can i say except a strong gut feeling i played a 2nd time despite my suspicions and same thing all over he played only slightly slower than chess.coms hard level computer on live and the little time i think would take just to look over at a computer screen and copy off the engines moves

If you suspect someone, please use link AnnaZafi posted above. When that page opens, submit a 'Suspected Cheat' ticket.
Also see 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
my most recent live game here on this a guy was using a engine while playing me but i don't know how to report him since its the first time someone ever cheated against me.