Hiding among plants and caves is usually one of the signs. The other one I know is the lack of appetite. That's as far as I had seen in my experience. The belly will also look like it is ready to burst. In some cases, some lfemale livebearers have black spot seen within them. The darker the spot near their anus is, this would mean that releasing of fry is getting near.
IMO, there's no need to try saving the fry. Livebearers are so prolific and with plants provided, you are guaranteed to have the strongest fry surviving whereas isolating the mother would mean struggling over trying to prevent all the fry from succumbing to illnesses and other factors that will easily harm and kill them.
IMO, there's no need to try saving the fry. Livebearers are so prolific and with plants provided, you are guaranteed to have the strongest fry surviving whereas isolating the mother would mean struggling over trying to prevent all the fry from succumbing to illnesses and other factors that will easily harm and kill them.