Sexing them is especially hard since most of the females have had hormones injected to change over to males because males sell better. But mostly like any other fish; females are a very dull bluish color (for the Dwarf's mostly, not sure about other species), anal fin is rounded not pointed, stuff like that. But it's extremely rare to find a female in the Dwarf species and again I'm not certain about the other species of Gourami since I hadn't looked that far.
But like most other species it's best to have a ratio of 1:2 M:F if not more females because like Betta's, they will pick on each other and hurt each other. So I'd really just suggest have one of the Gourami's, that leaves more space for schoolers anyway which will make them happy as well.
Leopards: 1:4 Hawkeye, Luna, Astrael, Dorian, Cullen