All female guppies will have a gravid spot. You can usually tell they're actually pregnant when they swim under a bright light, like an aquarium light, and the gravid spot isn't "empty" per se. You can usually see either the tiny eyes of the fry inside, or just a big black mass. If you see it being pink or empty, she is not pregnant, or not far enough along to be seen pregnant. If she is in a tank with other male guppies, or has been at some point, it's usually safe to assume she IS pregnant. The gestation period for guppies is around 28 days, give or take. Remember, fish can get fat just like humans, so just because she's fat doesn't necessarily mean she's pregnant. Hmm, I hope that made sense.