There are many ways to sex a Platy (these sexing methods, by the way, can be applied to all livebearers), but there are two that are more widely used than others. The first method is to observe the shape of the Platy, Females will be longer, and also have wider stomachs (even when not pregnant) than males. The second method (and most accurate) involves observing the three fins on the underbelly of the fish. Female Platy's will have three almost identical fins, two a little further up the belly (closer to the front) than the rear, and with a male, the two frontal fins will be the same, but the aft fin will be much smaller, and pointed.
I got this info from:
There are also drawings to show the difference.