Ahh, that's simple. Males are much more colorful, have a gonopodium (a rod-shaped fin near the back of their body), and females have a gravid spot (a dark colored spot around their bellies). Most females have very little or no color on their bodies but may have some on their tails.
Endlers can breed with guppies. Sometimes, the "endlers" in pet stores are just guppies that look like them.
They look like endlerish, the basic colouring but they usualy show swords on the tail and a large dorsal fin, how ever most shop endlers are already endler x guppy. Their are a few true endler's but these are still rare in the hobby.