Sexing mollies is easily done. They sex the same way as most common livebearers that you will run across at the Local fish shop. I( have pictures of somewhat less common fish that can show the difference easily. Once you have seen the difference you will be able to reliably sex common livebearers for yourself.
This is a picture of a male of a particular species that you may not often see in a shop but it shows the sex clearly. Note the shape of the anal fin, the one right next to where the fish eliminates its waste. On the male it is long and thin and is used much like the sex organ of a mammal to impregnate the female. That fin shape is called a gonopodium.
This is a female of the same species. She is showing the typical fan shaped (or triangular shaped) anal fin of a female.
The typical molly expresses their gender in the same fashion as these golden teddies.