How do I tell if I am playing an engine?

Of the 500 odd games I have played on chess.com, I have lost a healthy chunk of around 40-45%. Some losses you feel bad because you thought you could have won, sometimes you simply blunder. Many times the guy manages to get the better position and win. The kind of game chess is, you always feel pretty low after losing.

But just once in a while, say once or max twice in a hundred odd games, you find yourself feeling like you are being ground under your opponent's boot, and the guy isn't even very highly rated, just around your own or even lower rating. There is a sense that this is how it must be like playing against a grandmaster. Often, when this is happening, I have noticed that the time the guy is taking to move is totally unrelated to the posn on the board. Time doesn't seem to worry him at all, and he can keep you waiting even upto half an hour in a 60/0 game for an obvious move. The moves he makes, however, are crushing, and you soon start feeling overloaded with the possibilities they open up.

Overall, the issue is not significant, as 1-2% games only are like this in my view, but the experience is very unpleasant. It would be interesting if we could ask the computer it's opinion about such games. There is a facility of computer analysis; why not have another where the computer gives its' opinion of the chances that a particular player is consulting an engine, grading the possibility as, say, unlikely, likely or almost certainly etc.?