Sailfin Mollies, Dwarf Rainbow, Dwarf Gouramis

Livebearers, which include mollies, are the easiest types of fish to tell the genders apart.
Look at the anal fin - that's the one underneath the fish near the tail, close to where the fish poos from. If it is a typical triangular fan shape, it's female. If it is rod shaped, it is a male. This rod shaped anal fin is called a gonopodium.
If you look at platies in the fish profiles you'll see pictures of 5 fish. The two fish at the bottom have triangular anal fins and are female. The middle one and the two at the top have gonopodiums and are males - you can see it most clearly in the red and black fish at the top left.

But this only applies to fully grown fish. Fry all look female, and it takes a few weeks to a few months, depending on the species, for males to reach maturity and the anal fin to turn into a gonopodium.