If you purchased your guppy/ies from a pet store where it was in a tank with other guppies, it is probably pregnant. As far as telling for sure if it is or not, It gets easier the more time passes. The gravid spot, near the butt, will begin to darken and the stomach will begin to swell. Guppies will give birth approximately 30 days apart. There are some good threads on guppies in the forums if you want to dig a little further. Good luck!