Just call every bet, that way you can't be bluffed
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But seriously, most of the time against non-maniacs you don't have to worry about snapping off bluffs. Good situations though are when there are a ton of missed draws and villain bets out. Just like any other situation, you have to assign a range to your opponent. If you beat the majority (or have good equity before the river) you generally should be calling or raising. You'll never know for sure if they're bluffing, but bluffing usually can turn a marginal fold to a call, like say you're not sure your top pair 4 kicker is good but there was a flush draw on the flop that didn't hit. You should then be more inclined to call because a busted flush draw is now in his range.
So you really shouldn't worry about the one question "is he bluffing?" unless you are against a good opponent with a decent amount of history. If your hand only beats a bluff, you have to be damn sure it's a bluff before calling. The only real way though is practice and experience. Practice putting opponents on ranges. If either the range goes to 0 (ie no range makes sense) or is extremely top-heavy (ie you think he either flopped a straight or is on a bluff), then it's more likely to be a bluff.