Some butterflies show striking differences in both appearance and behavior between the sexes. The American lady butterfly (Vanessa virginiensis) is not one of them. Gender differences do exist in this species, but they're pretty subtle. You might be able to determine the gender of a butterfly that you raised from a caterpillar and are just about to release, but you’ll need exceptional observation skills to figure out if a wild one is male or female.
Despite the name, this species is not limited to North America. American lady butterflies are widely distributed throughout Mexico and the United States, but they are occasionally found in Central and South America, Canada and even southern Europe. There are usually two generations a year, sometimes more, with the butterflies overwintering as adults. Adults feed on the nectar of a variety of flowers, while the caterpillars eat the leaves of various members of the Asteraceae -- sunflower -- family of plants and others.
The primary visual difference between the sexes is that the abdomen of the males tends to be smaller than that of females. If you see two American ladies with noticeably different wings, there could be several reasons for it, but gender isn't one of them. The wing markings show variation between individuals and difference populations, but this variation doesn’t correspond to gender. Seasonal variation also exists, with the butterflies that emerge in the summer being larger and more vividly colored than those in the second generation, but again this has no association with sex.
A loitering butterfly is more likely to be a male, as males sit in wait for the females, often on hills, but this is not definite. On the other hand, if you spot one going purposely from leaf to leaf, stopping briefly and moving on, it's probably a female laying eggs. Check the leaf after the sighting for confirmation. If this was a female doing her business, there should be one tiny, yellowish egg on the upper side of the leaf.
Adding to the confusion are some similar species -- the closely related west coast lady (Vanessa annabella) and painted lady (Vanessa cardui), whose ranges overlap, are most likely to be confused with the American lady. They can all be distinguished by wing markings, with the most obvious being that the American lady has two eyespots on the underside of each wing, visible when a butterfly is resting with its wings together.