The benefit of the semi bluff is that sometimes you get folds, so if you semi bluff then someone raises big, you're usually forced to fold, especially if Villain has committed over 1/3 of his/her chipstack.
You can't just go on a hunch that Villain is on a semi bluff though, board texture and preflop play means everything.
UTG opens for 3x
Hero calls on the BTN with 7h7c
Flop comes 3c10s9s and Villain c-bets.
In this spot all the spaded Broadway combos are in villain's range for a semi bluff, plus all the KQ QJ KJ combos, also AJ, AQ both have back door draws so there's a huge chance that Villain is semibluffing here and a nice raise is a good idea since less than half of villain's range is beating you.
But if the flop came KQJ... Then the semi bluff is almost certainly not happening and you are likely up against a made hand.
Long story short, I like to ask myself what hands can Villain be semi bluffing
with? If there are more made hands than semi bluffing hands in their range, than that's what I go with.