There are writeups about this. The sites score each move, it is extremely unlikely to pick superhuman moves multiple times in a row. Once or twice maybe, but not most of the times.
Here, for example, is one of the more well known and easy meta-indicators of cheating: Humans spend more time on difficult moves than on trivial ones. Cheaters will typically spend a comparable amount of time on a trivial forced move, and the start of an exceptionally deep combination. And an even bigger tell is that said exceptionally deep combo may be followed by a couple of forced moves, yet he will again take just about the same amount of time to play those moves.