How can I tell male from female?
Depends on the species and the maturity of the fish in question.
Lots of monomorphic species have subtle gender-cues while other monomorphic species are near identical and venting is the only sure way.
In my opinion, though, particularly with
fully matured dimorphic species, venting isn't as necessary to determine gender. While some females of dimorphic species such as metriaclima lombardoi or melanochromis auratus may be able to feign male colors to some extent or another, the vast majority of dimorphic females cannot replicate the colors of a male in full breeding/full dominance color mode.
For example, a 5" metriaclima sp. msobo which is black with blue scribbles and blotches is a male, no venting required to acertain this (the solid yellowish females simply cannot assume these colors). With dimorphic species I think venting would come into play when attempting to determine gender of immature specimens prior to the males' color transition or when trying to identify sub-dominant males retaining the juvenile patterns which the females keep.
Metriaclima estherae (blue morph )/Red Zebra (Blue) take it a step further since males are born as gray-blue fish (not orange as their sisters are, thus no color transition from orange to blue occurs). Any gray-blue fry in the brood will for certain be a male (no venting required...
) .