It all depends on the type of finch. I googled "finch gender" and a site came up about how to determine the sex of a finch. But it gave step by step directions depending on type. Hope that helps. Sounds like it's kind of tough.
Thanks for the update. We have Zebra finches and white finches(nothing fancy-just the pet store models). I am trying to determine the sexes by checking out their orange beaks. Apparently, males have dark orange beaks and the females have a lighter beak. I just wasn't sure whether there was a better or another way to tell them apart. They are still to immature to tell by the beaks. They still have a lot of brown in them.
the male sings, and is also darker in color, I use to raise them myself
and when the beak turns solid orange, thats when you know they are ready to be taken away from the nest completely