You can seldom be completely sure, but can often get a better feel by thinking it through.
The guy limps then calls your raise pre-. What's his likely range? Note the word "likely" as well as the fact he just called.
He then check-calls a pot-sized bet on a very wet board. What's his likely range now? Again, note that he didn't check-raise.
The turn looks like a blank, and it seems like he check-calls another pot-sized bet. Same question again - what's his likely range now?
The turn completes the possible flush, and he shoves (for how much?) What's his likely range now, how much of it do you beat, and are the pot odds
sufficient to warrant calling even if you're probably behind?