All the major online chess websites have anti-cheat mechanisms. They don't publish details of how they detect cheaters though, and I don't know how good they are.
From what I've read, they work by comparing the player's moves against chess engines, and if the player is picking engine's choice too often in positions where there are multiple roughly equal moves, they get flagged.
I always found weird that someone would want to cheat in a game like online chess. I mean, what's the point? Does anyone have insight on what's going on in the head of cheaters?
A few reasons come to mind. One is simply that if you have any metrics (ranking, win/loss ratio, greater site access..) it's going to feel nice to see them improve. Another is that losing at anything can be ego-hurting (similar reason good players sometimes sandbag with new accounts / lower ranks they can't possibly lose to, they need to 'win' more). Or reverse sandbagging/trolling with a bot might be amusing. Another is the cheater may justify it as a self-teaching game, and might not always play the strongest move but see if their move is even in consideration or try to improve ability to see the better moves by having them always pointed out -- but why not just play the bot, or save that for post-game analysis? I like to run my go games through gnugo's annotated analysis at the end (as I'm very weak I assume even the weak gnugo can teach me things), it'd be too troublesome to use it in a live game.
it's where the enjoyment comes from. cheaters don't enjoy the game as much as they seeing their ELO/MMR go up or in the worst case they're psychopaths who just want to mess with other people's heads.
Even so most people don't bother with standard games online since its way too easy to cheat by mirroring the game and basically undetectable if they are good enough to not play lines that look like "computerish" moves.
From what I've read, they work by comparing the player's moves against chess engines, and if the player is picking engine's choice too often in positions where there are multiple roughly equal moves, they get flagged.