I've never looked at it as 'do I believe my opponent' if they bet. I look at it as 'what is my opponent betting with' and 'what do they want me to do in this situation'.
If I have 66 and the flop is J-8-2 rainbow and my opponent bets, I might think:
-are they betting with a J? possible-
if I raise, do they fold a J? no
-are they betting with an 8? possible
-if I raise, do they fold an 8? maybe
-are they betting with a 2? unlikely
-If I raise, do they fold a 2? possible but they don't have a 2 here.
-are they betting with a draw combo like 910, 710, or 97? possible
-if I raise, do the fold any draw combos? Possible. (This is more opponent dependent than anything.)
In this case, raising wouldn't do us any good as we don't want to get 3bet. We could be way behind in this spot or maybe slightly ahead. Folding is fine if you villain has it and you're drawing slim, but floating is also an option. What if the turn is a brick and villain checks. Then we can bet out and maybe villain folds their 8x hands and their draws. They likely call their Jx hands. Then what if the river is a K, A, or another brick, and villain checks to us again? We could check back here and go to showdown, but we only win vs busted draws. Or we could bet out here as 66 is only ahead of missed draws, but we can apply more pressure on the 8x hands and even Jx hands. And if they lead out on the river, we can just fold as we only beat out a complete bluff.