A male will have a smaller genital papilla.
As for ways to sex a labiatus, venting is the only sure fire way to do it. Once they get bigger you can judge sex by a few physical characteristics. Males will have a thicker body and generally be more aggressive.
As for citrinellus sexing this is about the same as labiatus. As they grow though the males will develope the classic naucal hump. Females will hump out too but not nearly as big as the males.
With the big box store hybrid midasXred devil it's really impossible to sex other the venting.
Hope that helps a little bit.
