IMHO, the only way you can tell is if the guy is tight and skilled. Say he bets, you raise and he pops you back. He can't have top set, so with this kind of player he should be scared of running into it. When he reraises, he's telling you he's not worried about it. But by this time, so much money is in the pot that you're going to stack off anyway. (unless you decide to call and wait for a safe turn) But to be honest, monster draws are rare, so usually I assume I'm not against one.