Mollies are more difficult to keep healthy and happy than other "beginner fish" like their close relatives the guppies and the platys. They have smaller broods than many live-bearing fish, but still breed prolifically. If you want to try your hand at breeding mollies -- or if you want to be sure yours don't breed -- you will first have to learn how to distinguish between their sexes. Several clues can help you tell male from females, whether they're balloon mollies or other species.
The easiest way to tell a male molly from a female is by looking at the anal fin. If you start at the back of the fish and move forward, the anal fin is the first fin on the bottom of the fish. In the case of balloon mollies with their rounded bodies, this fin points the same direction as the caudal or tail fin. In females, the anal fin is a regular triangular fin. In male balloon mollies, the fin is rolled into a tube-shaped organ called the gonapodium. If a fish has its fins clamped against its body, it might be hard to spot, but this is still the best way to tell male mollies from females.
In most livebearers, the body shape can help distinguish males from females. However, the distinction is less pronounced in balloon mollies. In general, female livebearers are more rounded than their male counterparts, particular at the rear margin of the abdomen. In balloon mollies, females do have more rounded bodies. But with these particular fish, it takes a bit more practice to tell the difference between a rounded male and a slightly more rounded female. You can use this as a backup for checking the anal fin.
Male mollies have larger dorsal fins than females. The dorsal fin is the back fin -- think of the movie "Jaws" if you get confused on this. Males use their larger dorsal fins for attracting the females. However, just like the body shape, the difference is less pronounced in the balloon molly than in other species and varieties of mollies. Combine this observation with those of body shape and anal fin shape to confirm your observations.
You can determine males from females by their behavior, too. Male mollies, like most livebearers, are constantly trying to catch the attention of females. The phenomenon takes the form of males swimming in front of female fish and flaring out their fins while shimmying. If you see a fish doing this kind of behavior, you have strong confirmation that he is a male and his immediate audience is a female.