vinnie
Legend
Silver Level
No, you should have a good feel for it based on watching him play, how loose he is, his range, and the flop texture. You will never know exactly how often a player will fold at that specific moment, with his specific holding, and against you. Maybe he would call against another player, but knows you will probably fire again and he won't get a cheap showdown. Maybe he will call more against you than another player because he sees you bet a lot and he wants to play sheriff. More importantly, you should be watching how others play and how they respond to bets on each street. Poker is a game of incomplete information. You have to use what you know and sometimes you might just have to take a risk. Generally, you should have a good feel for how often a player is folding on certain flops, turns, and rivers -- even if you have never played against that person before -- because you've played against many similar players and know how most of them react.
You also can sit around and run the math. If you put them on a range, you can figure out how many of their hands hit certain flops. You can do this by hand or with a tool like Flopzilla (I think this is the correct App, I haven't used it in a while). You can then try and determine what sorts of hands they continue with: say, "two overcards, flush draws, any straight draw, any pair or better, any Ace with a kicker higher than the bottom card on the flop" [this is pretty much what a lot of the really sticky players call with, right here].
Now, based on their starting hands, the flop texture, and your assumption/prediction of what they continue with, you will know how much of their range is folding to a bluff on the flop. The wider they call preflop, the more of their range they fold on the flop. Generally, you have a lot more fold equity here than you might assume. You might not be a favorite (50%+) to get a fold, but it's significant. Then work the same math for the turn card, assuming they may call a bit tighter, "two overcards, flush draws, 8 card straight draws, any pair" and see how much of their continuing range can call another bet. What doesn't call is your fold equity in the hand on the turn.
The river is a bit tougher. You have the whole board now. Obviously, all the draws that make it to the river fold, but you don't bluff if you have hands that beat a busted draw. How does their "calling range" on the river compare to the strength of your hand along with how much of their total range on the river can they call? If you have nothing, but most of their range should be weak and fold, you might venture a bluff. You might even go for a bluff if you can't beat their busted draws, and you expect a significant chunk of their range is busted draws. You can size your bet to make it +EV based on the size of the pot and your range estimation.
You won't have time to do that in the middle of a hand, but you can go back and look at hands afterwards and see. You should also try and develop ranges for players, as you play, and pay attention to what they showdown. Does it fit into the range you expected? The more you do that, the better you get at it. As you get better at putting someone on an expected range, you get better at knowing how much of that range folds.
tl;dr: I'll say it again. If you're not bluffing in spots that would be +EV--either because you don't recognize them or you're worried about getting called--you are making a mistake. You can survive making tons of mistakes at the micros, and even show a profit making tons of mistakes because you could still be making fewer mistakes than your opponents. This doesn't mean that it isn't a mistake.
No one is perfect at recognizing all the spots that are +EV to bluff. This is more along the line of Sklansky's fundamental theory of poker, except with knowing an opponent's specific range instead of his specific hand. If you knew exactly what cards an opponent might be playing in a spot and how many would call, how many would fold, and how many would raise... would you make a different decision than the one you did? If so, then you've made a mistake according to this idea.
