I did a random search and came up with this, I don't know anything about these guys, so I don't know how true it is.
""The male's fins are brighter red than the female's. Females are also more heavily built (rounded belly).""
""females are plumper, males fins are brighter.""
""males have red fins, females have yellow""
Seems to be pretty consistant everywhere. Hope that helps.
ETA: Well, not so sure about the yellow comment...
""The male will have brighter red fins, sometimes tending towards yellow. The female is fuller bodied and has a more rounded stomach.""