Are you saying they are mature when they pop off the plant ?
Yes. You know what seeds look like, right? Usually the darker they are, the better. Black seeds, or brown seeds with black stripes on them are what you want to see. Also, look for a little dimple in the top, where the seed was attached to the plant. The top of the seed should look kinda like a volcano top and the other end should be nice and pointy. Im not sure if it is just my exp. Or whether i have just been lucky, again and again, and again, but when the top of the seed has a dimple in the top, they aleays turned out to be fems but whenever the top flat or had a slight bump on it, they always grew up to be males. It is possible that I jave just been lucky but since i have used this method to pick out 200 female seeds out of a whole plant, and they were all fems, i would say that statistically, this works. You could always wait until 3 weeks into flower and mess with the light schedule on one plant o hermaphrodite her. Once she grows balls, you can use that pollen to pollinate your other females and you will end up with FEMINIZED seeds. I hope at least some of this is useful to someone.
Happy growing
DwJ