Not gonna lie, I'd smile and might not be able to stop myself from flexing openly enough to look like an imbecile.
Native Italian speaker here. "Forte" does not only mean Strong, but it's also slang for "cool"! If you are forte, you probably make good jokes, are intelligent and are considered a good company.
e.g. Marco: Why did the chicken cross the road? To get to the other side!" Lara: ahahah Marco sei proprio forte!
Here's a scenario. You're going on a huge trip around Sicily. A beautiful, sexy, Southern Italian woman's vehicle has stopped functioning. You help her move it from where it was. If she just says, "Grazie!" she's thankful. If she uses this you know that you've charmed her.
Under many circumstances (including the "flirtation" training), "Come sei forte!" has nothing to do with physical strength. (Unless, of course, the man or women is lifting weights to show off or something silly like that.)
Forte is used as a way to say amazing, or incredible, or brilliant. If a person says, "Come sei forte!" it's meant as a compliment to your character, or your humor, not your physical strength.