I don't pretend to know anything about these fish (though this thread has piqued my interest and now I'm reading about them).
I found this on wikipedia;
Sexing of juveniles is impossible because these fish "choose" their sex as they mature. Once one puffer begins male he excretes hormones to prevent the other puffers from becoming male. However, if two fish start to mature into males at the same time one will become the dominant male.There is also a dorsal crest, but it lacks special colouration when not erected. Both crests are displayed during courtship while the male circles the female. They also will have more yellow colouration.
Both sexes are primarily yellow with dark green to black iridescent patches on the flanks and dorsal surface, but as with other members of the genus, sexual dimorphism is apparent in mature fish, with males being more brightly coloured than females. Males can also have a dark stripe down the center of their pale belly and iridescent "eye wrinkle" patterns that females do not have. Females are more rounded, tend to be a bit larger than males, and may or may not show more smallish spots between their larger dark markings.