Molly's gender is determined either by their dorsal or anal fin. If it's a sailfin molly, the male will have a really BIG dorsal fin and also the male will have a "stick-like" fin underneath him, whereas a females anal fin will just plain look like a normal fin.
Female mollies take about a month to give birth, during that time their stomachs will get fat(naturally) and then it will begin to "Square off", which means that the body just looks more square shaped. When the babies are almost ready, you'll see an "eye spot" which looks like ..an eye. When they're a few days to a week before birth they'll get really fat again. You'll definitely know when they give birth though, cuz they'll be really thin o.o and the babies will probably hide on you.
