I got a notice from chess.com that one of my opponents had violated the Fair Play Policy, and therefore they were adding 16 points to my rating, which doesn't matter that much because chess.com ratings are always changing. They didn't tell me which opponent was the culprit, but I was extremely curious as to who it was, so I looked to see which one of my previous opponents had his account suspended.
It was from an online tournament that I was playing, which makes me wonder if they only check tournaments for cheating. It seems to me that it would take way too much computer power to check the millions of games that are played daily on chess.com.
Oddly enough, this was a game that I lost because my Comcast Internet went down in the middle of my game and stayed down for a couple of hours. What I heard was that the service was down for the entire midwest. This was particularly painful because my opponent was only rated 600.
One way they determine if someone is cheating is to look at the accuracy of their moves with computer analysis. Anything over 98% is probably suspicious, especially if a low rated player does it on multiple games.
The accuracy of my games tends to vary from 60% to 99%, with many games over 90% and the vast majority over 80%. I have a few occasional games that are 98 to 99% accurate and I am sure that chess masters do too. I hope that if I were to have a really good day that they would not think that I was cheating.
I read that cheating on backgammon web sites has mostly been eliminated by checking the games with computer analysis.
Maybe ideally they would use webcams to watch everyone as they play. It is pretty hard to cheat if they are watching you, or even if just your opponent can see you. It's not very private, but they do this for some online matches with top-level players.