guppy fry

They tend to be hard to tell apart, you should seperate them if you are breeding them. If you do not care about them reproducing then they can live together. As for how to tell them apart the females have a full fin on the bottom and the males bottom fin is more like a rod. Take a look at the pics here http://aquariumhobbyist.com/guppies/anatomy.html

But from what a lot of people say it is hard to tell them apart at a young age and some will slip through.