In general i would say, that unrealistic laptimes is a sign (after a few years the top drivers have set a "limit" on each track). Also unrealistic high corner and straightaway speeds or an impossible acceleration can show you that something is wrong. I think you can also see in the program replay analizer if someone uses the so called "weather cheat" in case you get his replay (you can also see corner g-forces etc. with that program i think). I might be wrong on that...