This article will tell you exactly how to tell the gender of your molly fish.
Mollies are a very popular aquarium fish. They are very hardy, come in many different varieties, they are active, and they have great personalities, but they breed like rabbits. You buy just one of these fish from the pet store, and you will probably find lots of babies in your tank once a month. You may ask yourself how it is possible for just one fish to have babies, or if you have several of them, which ones are having babies. Molly genders are not as easy to tell as guppy’s but you can and here is how. Male mollies have a pointed fin near their anus. This fin will be very narrow, and it acts as his penis. Here is a photo: http://upload.wikimedia.org/wikipedia/commons/1/1b/Black_Molly_3_Male.jpg While female mollies have a more rounded fin that is larger. Here is a pic: http://www.bbayaquariums.com/black-molly.jpg
As you can see there is a large distinction in the two genders. You just have to look. If you are planning on breeding them. You should have at least two females for every male to cut down on stress. Also, mollies will eat their babies, so you should by a breeders net or a three way breeder to put the babies in.