How do you tell if a pacu fish is male or female?

There are several species known colloquially as Pacu fish. They are all related to and larger than the Pirahna. They get VERY BIG. Over a metre and weighing in at over 60lbs. To the best of my knowledge there is no sexual dimorphism in aquarium specimens. I would assume that in adult fish the female is the one that lays the eggs.