In general, it is safe to assume if your female has ever been in the same tank as a male, that she is pregnant. Once a female mates once, she does not need to mate for another six months. The average gestation period is 28 days, give or take a few. When her gravid spot, the blackish brown spot at the base of her tail, is very large and squared out from the back, and her stomach is so large she looks as though she may burst, it is a good idea to separate her from the rest. On top of that if you have never bred guppies before they are live bearers and deliver their babies slowly and if you do not separate the female from the other guppies, and her from her babies, the newborn fry may become a nice snack. If you do not have a breeding tank or just a breeder, the fry will be safe if the tank is heavily planted, but you might not get the full litter. An average "litter" as I say, can range from 20-30 on average, plus or minus.
After the babies are all born, if you have males, keep the female away from them to rest so they don't immediately harass her again. As for the care of the baby fry, they especially enjoy baby brine shrimp, but I personally have gotten away with feeding them my normal tropical flake food, I just crush it to almost a fine dust for the babies. For filtration a sponge filter will work. Be sure to take out any sick or dead fry so they don't get the rest of the stock sick, and watch them grow and enjoy your new stock of guppies! However if you don't want even more babies, be sure to separate the males and females as soon as they are mature enough to decipher the gender.
Male guppies have a rear fin that is rod shaped and usually have larger and brighter tails, whilst females are (usually, not always) duller and have a fan shaped rear fin.
Hope all this will be of use in the future if your guppies ever show signs again
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