Online you may never know if you have been bluffed, if the player in question mucks their cards. What realy hurts is in a monster pot you have chased, but folded to a massive reraise or all in. And then they show you bogus cards that you had beat hands down. Now thats a bluff with a little salt in your wound.When playing cards, how do you tell when someone is bluffing?
I own a crystal ball.
Honestly, you will almost never know that someone is bluffing. You will frequently however have enough information to know that the range of hands that someone is likely to play that way contains enough bluffs to justify a call.