https://support.chess.com/article/648-what-do-i-need-to-know-about-fair-play-on-chess-com
There are a number of methods but the site does not release details of cheat detection processes.
That said, discussions of cheating, potential cheating are not allowed in the general forums. If you like to discuss, join the following club:
https://www.chess.com/club/cheating-forum
I've received periodic messages from Chess.com indicating that they've determined that someone I was playing has cheated.
I'm curious as to how they figure that out. I don't know HOW the opponent cheated so I'm unsure how chess.com figured it out. I suppose I'm curious because I'd like to know if there's a way I can determine is someone is cheating.
Honestly, it's pretty petty to cheat on games with me. I'm a high 700 level player far from a formidable opponent. I'd simply like to know how they knew.