I might also suggest as a long term GM that worrying about cheating in an environment of collaborative storytelling is unnecessary to begin with. If you're truly worried about a certain player fudging their rolls constantly to the detriment of other players, then that person likely shouldn't be a player to begin with. I seriously suggest just rolling through without worrying about 'cheating' too much, most players aren't actually interested in it, and it's not once in my life ever been an issue when I couldn't see my players rolling, nor has it affected anyone's enjoyment of the game.