Dave is right about the 2-3 females per male, but I don't think you would have enough room in your 30g with what is already listed. You are overstocked if your 2nd 30g (the goldfish one) listing in your signature is current, and your first tank shouldn't hold 10-15 female guppies (based on the number of male guppies your signature lists.) If the males and females are sexually mature (about 3 months of age) and are in the tank together for more than an hour or so then they've bred. Guppies are like the rabbits of the fish world.
The females will develop a purple crescent moon shaped splotch on the back end of their belly, that is their equivalent to a uterus and is where the fetii will be held. They will bulk up in the belly area as their 28-32 day gestation period gets further along, and near the last few days will kinda get a square shape to the belly. (Looks almost like they swallowed a box.) A lot of livebearers, guppies included, will eat some if not all of their fry sometimes. (Some of our 20 something strains of guppy will eat their fry some just don't eat them, no explaination.)