Guppies are great pets -- they're small, colorful and get along well with most any other fish you could keep them with. In order to breed them, you will have to learn how to tell the males from females. Fortunately, there are several telltale signs.
Size
First, female guppies are larger than the males. When they are the same age, females may be as much as twice as large. Not only are they longer than their male counterparts, but they are more heavy-bodied. Females can be larger than two inches, while males may only grow an inch in length. The size is the first major hint to your fish's sex.
Color
In general, male guppies are much, much more colorful than their female counterparts. Female guppies are typically a silvery, drab, pale gray. Males, depending on their breeding, will have colorful fins and bodies, including colors like reds, oranges, blues, greens and yellows. This contrast is less obvious in "wild-type" guppies, those that resemble the original wild guppy, but very obvious in the highly bred "fancy" varieties.
Anal Fin
The biggest, most certain way to tell if a guppy is a male or female is to look at the anal fin. This fin is on the bottom side of the fish, near the tail. It is between the paired ventral fins. In the female, the ventral fin looks like any triangular fin. In the male, the anal fin is "rolled" into a tube-like reproductive organ called a gonapodium. This is the most accurate way to tell male guppies from female guppies.
Gravid Spot
Another good way to tell a male guppy from a female is the presence or absence of a gravid spot. Pregnant female guppies (and female guppies are almost permanently pregnant) have a dark spot just above the anal fin. This is the developing young guppies. As the brood gets further along developmentally, the spot will get darker and expand. Males never have this spot. It's also a good way to tell if your female guppy is pregnant, and how soon she will give birth.