I just aquired a friends breeding pair of banggais (he is moving to germany). Even though they are a known pair, I could not tell the difference between them till the males jaw expanded. The middle of his jaw will drop a little, kind of like a second chin so to speak. Anyway, he swallowed what I figure was a new set of eggs.
But like I said, couldn't tell the difference. In fact all the "potential signs" were backwards. The female is just a hair bigger, her fine is longer, there is no clear "angle of swim" difference (it was supposed that a male floats angled down (head below belly) with female swimming angled up), and I can't see any difference in the rounding where the tail meets the body.
But, I have noticed this... they act different. The female is way more skitish. She hides in a cave when startled. So maybe the key is not in how they look, so much as in how they act.
Hey Dr. Marini, maybe there is an idea for a study, "Distinguishing the Sex of Banggai using Behavioral Queues".
Well, its an idea I guess. Good luck, Kevin