Everted ovipositor is only really visible in well-conditioned fish close to breeding, and isn't a very certain indicator to the untrained eye. Venting would be a possibility in mature fish (i.e. flipping it over and looking at the vent size and/or shape), but I'd be hesitant to consider that reliable in a fish that looks as young as yours. It is said that female flowerhorns have more broken patterning and a black blotch that extends into their dorsal fin, but I've seen a couple proven females without said blotches or extensive spotted pattern on their dorsal. The wacky, nonsensical genetics of hybrids just makes it real difficult to accurately sex them based on pattern and whatnot. I'd wait a bit to vent as well, as it likely won't be very accurate until the fish reaches or is close to sexual maturity.