Livebearers are usually always pregnant.
If there is a male in the tank (or if they were kept with males at the pet store) it is likely that the female is pregnant. Of course, the first sign that a female is pregnant is the growing abdomen. Another sign would be an increase in the color or size of the gravid spot
(the darkened spot near the anal area)
A few signs that your livebearer may be ready to give birth to fry are , (1) she may stake out an area in the aquarium to give birth (2) she may also stop eating and act very shy and hide from you and other fish and (3) her tank mates may also follow her very closely.
(I find this to be true with my platys) Also, the abdomen will appear very large and get a squared box look to it.
Good Luck.
