You have to pay to see if they bluff you. Be it online or live. And you will never know for sure unless you have the nuts.
But there are some indicators. Other than counting probabilities its combining hints like someone who see and bet a lot of flops, someone who caught at showdown betting big with marginal hand, someone betting from the button, someone whose bet seems to try to put you out of the pot, someone who folds when check raised, someone who can afford bluffing, when there is a few players in the pot or its heads up, when you give spot for to be bluffed like showing weakness.
Most players go Tom Dwan when catched bluffing on showdown and exploit their loose table image. So showdowns can be tricky. My personal favorite is when I fold preflop the nuts and see someone betting the river like he have the nuts. Nobody knows if he really had it or not, besides me.
In conclusion, bluffing is telling a story. How do you know when a story is fake? When you find irregularities and things that don't match when put together. Betting that dont support the story should ring you a bell. If you hold the nuts would you bet the turn big enough to make your opponent fold? No, beacause you want them in the hand. Sometimes a check raise works for me to make someone fold a bluff. Being a little calling station like Negreanu makes bluffing you riskier and would go to more bluff-susceptible players.