As long as there is money involved I don't think cheating will be out of the option since some people will do everything even cheating just to have the wagered money or the pot money. The OP was looking at some gambling site for a p2p chess online which I don't think exist yet.
ya.ya.yo!
Based purely on my personal experience watching online tournaments live, cheating is almost non-existence if the players that consist of such are within the top 20 of the rankings. If those GMs that are within the top 20 participates in a tournament, you can almost guarantee that they will not be using any chess engine during the whole match due to the effects of it on their careers.
I would somehow understand if the players involve are within the range of 1700-2500 but when a tournament consists of players within the top 20, then I guess it is safe to bet on those games. I just hope that there will be websites that will consider chess gambling as it is really an underrated game to be betted on.