How do they know this person cheated?

I lost a game to someone who may have cheated, https://www.chess.com/member/qhujt

There is not a problem mentioning them because chess.com has already closed their account. I am just curious, which game or games do you think they cheated in? When I look at the games, I can see mistakes/blunders we all made, and I don't see how this person cheated. They just took advantage. It's not like they played with small percentage advantages. In my game for example I could have played Qf6, instead of pushing a pawn. Seems like that is not that out of human thinking.

So, how do they know this person cheated?

There is no need to lock this thread, the person's account is already closed. We are not questioning if they did it anymore, but how chess.com knows it or is convinces enough that it happened. If it were a change in evaluation gradually, I might understand, but the opponents (me included) made significant human errors which another human could capitalize.

It's not like they played questionable pawn moves and then trapped pieces in hard to figure positions. I have played other people on other accounts who have been on here for years and played hard to explain things and won without ever reaching 1000. It raises the eyebrows.