Society finches are monomorphic; although some claim to be able to tell them apart, realistically there is no way to tell males from females just by looking at them. Instead, you have to study their behaviors. Given half a chance, and a female nearby, a male will almost always sing his Hunka Hunka Burning Love song, and dance his Hunka Hunka Burning Love dance. A serious breeder of Society finches once described his method: he took a known female and put her in a cage by herself. Then, he took an "unknown" bird, and put it in a nearby cage. He'd wait for a day or two and watch: singing and dancing meant a male, no special reaction meant a female. If you're serious about breeding any bird, you need to be sure not only of gender, but also of blood lines. This breeder would then put a band on one of the "discovered" bird's legs that not only visually told him which sex the bird was, but also helped quickly identify the bird's family heritage. That way he wouldn't let related birds breed with each other.

The National Finch and Softbill Society sells both closed bands (put on baby birds to prove they were raised in the US), and open bands in 10 different colors and five different sizes, which can be used to help identify individual birds. (Frank and Sally arrived with open bands, but they came off after awhile.)

If you look carefully at this photo of Bosco (on top) and Goober (in the box), you can see differences in the coloring of their feathers. Bosco's head and top beak are much darker. These birds are both females, but the sorts of differences you see here are exactly the kinds of things I have to look for when I'm trying to tell all my Societies apart. These differences are subtle, and particularly hard to discern when it's dark, and I'm trying to catch them.