I think the problem here is that chess engines are going to recommend 'non-human' moves at any point of the mid/endgame. And these 'non-human' moves could be the first/second/third best moves according to the engine. You'd have to have to have decent understanding of the game to realize what moves aren't going to blow your cover. I am confident over the long run anti-cheating software could pick this up in most cases. But also what are the incentives for someone to cheat at online chess over the long run? It wouldn't be that interesting for more than a couple days to get a quick thrill of beating a bunch of people in my opinion.