Your only other choice would be to set up another tank just for the females and then once the fry are born move them back into the tank with the males. This is always how I've done it if I wanted to raise all of the fry, but even when I left the fry in the tank with the adults I still had quite a few survive. Moving the females when they are pregnant can stress them out to the point of miscarriage or of holding their fry which could eventually lead to death so if you do plan to move them you need to be sure you do it as stress-free as possible. You could add a few more hiding places in your current tank and that would increase their odds more if a separate tank isn't an option.
As for when I female is about to give birth, there are a few signs that I've always noticed. Of course, the female's gravid spot will be large and quite dark. Mine have usually been really restless, swimming up and down the glass frequently. She will probably rest on the bottom or on a leaf when she actually begins to give birth but I've seen a couple of mine give birth while swimming. I've read that they stop eating before they give birth, but I've never seen this out of any of mine. Good luck with your little ones!
