You can look at moves as a series of probabilities. For each move, classify if it's more a blunder or inspired move and then look at people's games and see if they consistently have 1-2 moves that are much much better than their typical.

But what if they only cheat occasionally? A top player would only need a handful of moves to go from say 3rd in a tournament to winning.

The problem is that at that level they're more likely to make the absolute perfect move than not. Super GMs often play 95-98%+ accuracy games.