As it's said there is no more pleasant feeling than catching someone's bluff. About the question. It mainly depends on the board . For examle your opponent didnt bet during all streets. But he suddenly bets the river. And if the river card doesn't change anithing towards the board your thoughts must be like: why is he betting? Then you watch the size of the bet . If it's of big size that can mean two things: either he really has a good hand or he doesn't want to show his hand. So you will learn the things by experience. Sometime you may call till showndown because of curiosity.