When an OOP villian does the Check-call, check-call, big bet, this is a sign of a monster. He checked the flop and called your 2/3 bet. OK, he may still have nothing. Then you bet 2/3 pot and he called again (after checking). You should start to suspect something fishy here. The final tell is that he went from passive to agressive all of the sudden and he bet out instead of checking--basically telling you "I flopped a monster and was reeling you in and now I want to make sure you don't check to end the hand". Classic monster flop tell, especially since he didn't go "all-in" like most donks do when they miss their draw.