computeropponen:
Well, you can try to take some chess engine (e.g. Houdini or Stockfish) and see if some player does the same moves as one of this engine most of the time. If yes, the chances are high that this player uses this engine. On ChessBase server there is even automatic cheater detection, which does something like this.

If you really want to make sure your opponent is not cheating, play some chess variant, like Dark Chess or Embassy Chess. There are no engines for Dark Chess yet and for Embassy Chess chess engines are fortunately not that strong.