The so-called 'gravid spot' is of no use in determining pregnancy in livebearers, I'm afraid; it's more likely to be part of the fish's digestive system.
Long, clear poos are also of no use; they could mean the fish is about to give birth, but equally likely slightly constipated, has gulped a lot of air at the surface while feeding, or even that it has intestinal parasites.
The only true way to tell if a livebearer is near to giving birth is if she's 'squared off'; that means her belly has developed distinct 'corners'; it will look like she's swallowed a box, but young females might not get very boxy, if they haven't had many broods.
If you're thinking of removing the female to some sort of breeding box or net, then please think again. Those boxes/nets are too small for the female to feel comfortable, which can lead to her aborting or eating the fry, and they're too small to grow fry on in.
Adding plenty of hiding places to the main tank (fine leaved live plants, like elodea or cabomba are best for this) is the best way to ensure some fry survive.