Sexing common livebearers is easy as can be. As an example I will show a male and female of the same species, not one you are likely to have but the sexing is the same.
This is a male as you can see by the long thin lower fin on him.
This is a female of the same species as you can tell by the triangular fin. In this particular species the spot on that fin is also pretty standard for a female but is not the thing to look for.
Sorry about the grainy picture quality but as you can imagine, a fish only 3/4 inches long at full grown can be a bit of trouble to get a clear picture of.