How do you tell male guppies from female guppies?

There are many differences not the least of which is the size difference. The females are much larger than the males, and the males are much more colourful than the females which tend to be quite drab. The actual sexual difference can be observed by looking at the fishes Anal fin which in the female is triangular but in the male this fin has developed into a pointed funnel (called a gonopodium) along which his sperm (milt) will travel.