Comment by retsibsi

The most serious tournaments are played in person, with measures in place to prevent (e.g.) a spectator with a chess engine on their phone communicating with a player. For online play, it's kind of like the situation for other online games; anti-cheat measures are very imperfect, but blatant cheaters tend to get caught and more subtle ones sometimes do. Big online tournaments can have exam-style proctoring, but outside of that it's pretty much impossible to prevent very light cheating -- e.g. consulting a computer for the standard moves in an opening is very hard to distinguish from just having memorized them. The sites can detect sloppy cheating, e.g. a player using the site's own analysis tools in a separate tab, but otherwise they have to rely on heuristics and probabilistic judgments.