When I was first learning about fish I was told "Endlers and convicts... keep em wet they breed" I know that isn't actually the case, but not too far from it. They should have the gravid spot most livebearers get, as well as enlarging around the abdominal area. But they pretty much stay pregnant once they are sexually mature. I think.
Livebearers are usually always pregnant. ;D I have some female endlers that I am hoping will give me some fry.
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 may show you that your livebearer may be ready to give birth to her fry...like... she may stake out an area in the aquarium to give birth, she may also stop eating and act very shy and hide from you and other fish. Also, 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.