Livebearer females tend to STAY pregnant at all times after hitting sexual maturity if kept with fertile males.

The female swordtails, platies, guppies, and mollies tend to have a 28-32 day gestation period. Which means when the female finishes giving birth she uses the sperm she stores for upwards of about 6 months to reinsemenate herself and she should, barring the occurance of several possible problems, give birth in 28-32 days.
Livebearers often eat their fry and in a community tank it is even harder to find fry before they are devoured. Usually the females begin to bulk up in the belly area, as well as showing a gravid spot which is often purple, black, or clear (showing as pink due to seeing insides) that is shaped like a crescent moon. The females tend to either go to the surface or the bottom or the tank (not hang out in the middles as much) when a day or so away from giving birth. They tend to appear to be hyperventilating for that period of time also. That means that they pull extra water through their mouths as well as normal amount through the gills, gathering all the oxygen they can.
The pectoral fins also seem to be flapping a lot faster than normal when close to and during the birthing process.
I hope that helps some.