I have answered the question of is it male or female so often that I finally gave in and posted a pinned topic, pinned it myself,
here. It is a short description of how to tell a male anything from a female of the same species. It includes both how to tell a poeciliid male from a female and also how to tell a goodeid male from a female. Unfortunately, I had to use examples from among my own fish, so I could not show things like hifin swordtails in the mix. Those can be harder to tell male and female apart than the more common livebearers.