“Zakochać się” is “to fall in love” (e.g. a one-time ation), so the most literal translation would be “How can I tell if I really fell in love?”
So just like “I fell in love” in English, it might be interpreted as “I’m currently in love”, but it doesn’t have to mean that—all depends on the context.
If you wanted to explicitly say “to be (currently) in love”, you would use “być zakochany(m)”, so the sentence would be “Skąd mam wiedzieć, czy naprawdę jestem zakochany?”.
The word “zakochany” is a passive participle formed from “zakochać (się)”—so it pretty much works like “infatuated” or “enamored” in English.