I have always gone by how dark their gravid spot is. Last week, I was looking at her, deciding when I should move her into the net breeder when suddenly a fry popped out. Quickly moved her. Obviously she had several before that as we now have about 6 living in the big tank with another 20 or so in the net breeders (2). Will be moving all fry to their new tank this weekend. My mature female is always fat, even after having 50 fry!