Sometimes I feel that a human, particularly a human of about my elo rating, would not ever play so.
I never used an engine in my life for cheating, but I think it is not so hard.
I think, most of the cheaters does not automatize "well". Thus, he has no way to step with the engine on the spot, instead if he switches to it, he needs to duplicate the state. This gets some time - maybe a minute - for him. After that, he starts to play astonishingly well.
If I experience this pattern, I report it, but there is a bitter feeling. Obviously reporting someone beause I lost is no good. I think I should do this only if I am sure, but I can never be sure.
There are rumors that chess.com has some very sophisticated technology to detect engine cheaters, but I doubt that they could really do it. I believe that they do their maximum (like utilizing some AI to measure the "human-ness" of the steps of the players), but I think anything what they could do, can be easily gamed (like using the engine only for some, critical steps of the party).
Do maybe some other patterns exist, which could raise the doubt?