As others have said, sometimes a check is weakness, sometimes they are just hoping you will bet at it. (I often check my monster flops if I am in early position... and, boy, do I love the guy who makes a move on the pot in late position. )
It's not necessarily a bad move, that bluff when you sense weakness. But don't do it much, and be very careful putting too many chips into your bluff. Some tables, a mere min-raise is enough to take down a pot. On other tables, a pot-sized raise is needed to show you are serious. Obviously, if you are walking into a trap, you could lose a chunk at the second sort of table because you have to fold to that check-raise.
Oh, yeah-- another point. Don't forget to fold when your bluff is raised. Do not forget it was a bluff. (Some of you will snort at this -- others will know exactly what I am talking about. I know I've done this. I have gone to some lengths to stop that expensive leak. When I bluff, I say to myself: Hmmm, I wonder if anyone has anything? And the bet is that question in chip form. A reraise becomes the answer to that question, and my fold is: Oh, so you do!)