The males are the really colourful ones with long tails and patterns, the females are usually duller and not much colour with short round tails.

Here are some pictures of males.
http://www.aquarist-classifieds.co.u...image/6445.jpg
http://homepage.ntlworld.com/g.skuse.../guppymale.JPG
http://www.aquariumtropicana.co.uk/I...ppy%20male.JPG

And here are some females.
http://animal-world.com/encyclo/fres...fe_guppy_s.jpg
http://www.aquariumfish.net/images_0..._from_kate.jpg
http://www.englishare.net/literature...ppy-female.jpg

They will usually have them in different tanks at the store which will be labeled, so it shouldnt be hard at all to get all males. They come in loads of different colours and patterns as you can see on those links i've provided.