Use the in game analysis feature to see if all their moves are "excellent". Also look out for random King moves that you don't understand.
How to detect engine?

@winny well why u say me coz i want to report the engine user and detect damn engine user ofcourse. There are many technology available there to analyze and see engine variations per move like with chessbase reader software...So i need to know correct ways to find out.
I feel like when you're close to 2000 blitz you can tell on your own in most cases... if during a brief after game analysis you can't find a single improvement (just on your own) for your opponent, and they're not titled, that's when I suspect cheating.
Engines choose the same move in enough positions that you don't need to check with 3 (or more) different engines. When I suspect cheating I set the engine to show its top 3 choices and click through the game fairly quickly and see how many times my opponent's moves agreed.
This is a pretty simple method. You can't be sure without more games... but you don't need to be sure to report someone. Chess.com wont ban a user without checking their games first. Chess.com's method is more thorough and isn't public knowledge.

You can look for long forcing combinations. Also engine users spend pretty much the same amonut of time on every move. For example. A simple take the queen move can take just as long as some tricky exchanges. They usially spend about 5 seconds on every move. If it is a blitz game.
I wonder what satisifaction the cheaters get from a game they didn't play.
I think the motivation goes something like: it's not much fun to cheat, but it's even less fun to lose.
I wonder what satisifaction the cheaters get from a game they didn't play.
I think the motivation goes something like: it's not much fun to cheat, but it's even less fun to lose.
How can you stop someone from running down or freezing the clock.
Many times I'm 2 moves from winning a game then all of a sudden even though my time shows more than my opponent i end up losing.
Time to stop the cheats

Discussions of cheating are not allowed in the general forums. If you wish to discuss, or find the processes most often used in detection, then join the following group.
https://www.chess.com/groups/home/cheating-forum
Also, if you suspect a player of violating the rules you can report them:
https://support.chess.com/customer/portal/emails/new
Finally, @kingmena, lockups are not caused by your opponents. The chess clients don't directly connect. If you are having those symptoms the problem is likely on your end.
Disable Flash for the site and follow some of the suggestions in the following. It is about disconnects but many of the suggestions can help with slowness and lockups.
Want to know how can i detect engine if i doubt that my opponent used engines in the finished game with me?? i found database.chessbase.com where 3 top engines show only one variation per move, but it's not enough, so tell me anyone how to find that whether my opponent used komodo, stockfish, fritz or not?? and what are the correct ways to see that. Thanks in advance.