You can assume that all female livebearers are pregnant, all the time. Just watch for her getting fatter. When she's within a day or so of giving birth, she'll 'square off'; that means her belly will develop a boxy shape.
You don't need to do anything special; make sure the fish is well, but not over, fed and that the tank has good, clean water. Make sure there are plenty of hiding places in the tank, especially near the surface, if you want some fry to survive. Fine leaved, live plants are best.
We do not recommend the use of breeding boxes; they're too small for the mother fish to feel comfortable, and too small for the fry to grow properly.