The way to detect an obvious cheater is when somebody rated like 300-400+ rating points below you, a matchup which generally only occurs in tournaments or the like (normally you wouldn't be paired down to such a large extent), plays incredibly well and beats you. And then you do computer analysis and you see they played near flawlessly. You look at the move timing and make really complicated and deep moves instantly. Then you look at their game history and see they played their last few games nearly perfectly. Then you report them and your rating points are refunded a few minutes later.

Lower rated non-chess players, the kind that are most likely to cheat, can't just pretend to be higher rated people because they don't know what moves are suspicious and which ones aren't. And if you think they'll just cheat for the first few moves and then turn the computer off... it really just doesn't work as well. If they actually have no idea what they are doing they'll still lose, and it's super suspicious anyways based on move timing and computer analysis.

And to my original point, if I were an 1800 player and wanted to sneakily cheat to be a 2000 player, I probably could pull it off, because that is close enough to my skill level that I could pretend. Maybe by using an opening book to make sure I'm getting great positions out of the opening. Maybe by only turning the computer on during certain key positions. But again, if I did this consistently, I'd get a 2000 rating and now I'm going to be matched up against 2000 rated players, and from their perspective I'm just a 2000 player. And what have I gained? I now have to cheat to even be competitive in a casual chess game, whereas if I just had my true rating I could just play normally which would be easier and more fun. This is why sneaky cheating just isn't very common, because the only people capable of doing it really have no incentive to do it. The only people who think cheating would be fun are the people who are easiest to detect.

A few signs:

- taking a while to make an obvious move, especially in the endgame, where there might actually be only one move that makes any sense to make

- taking a very consistent amount of time between moves

- shuffling pieces around in a way that doesn't really accomplish anything but also doesn't cause anything bad

- playing normally, then a big pause and they go offline for a bit, then come back and start playing much better (connecting the client to an engine)