IMO, venting is a good method for species of fish in which both the males and females look the same when mature.
For haps, IME if they are large enough to vent with 100% accuracy, they are large enough to visually study and reach a sex conclusion.
Looking at the first photo, the fish you have is a male F. rostratus. The flecks of color in the dorsal fin give it away. If you look hard enough at the caudal fin, you can also see specks/dotting of color coming in as well.