A key fact to understand in thinking about cheating in over the board chess: a strong player can defeat a much stronger opponent with just 1-3 hints per game indicating the strongest move. For example, most chess experts agree that a ~2600 rated player with 2-3 hints at key moments per game would be expected to beat a ~2800 rated player. Many people might assume that a cheater needs guidance every move, thereby requi…
Chess tournaments could use the services of already existing casino anti cheating experts. But I imagine that would be very expensive and not feasible for most events.
I can't think of any effective way to curb online cheating in chess. Ultimately, online chess with money prizes shouldn't really exist.