They will have babies there is no way around it. You'll just have to deal with it until it runs out of sperm. They will eat the fry though so you shouldn't really wind up with a bunch of extra fish.
As to sexing, I'm not 100% sure it applyes to mollies but with guppies and platies you can look at the anal fin(The last fin on the bottom of the fish just before the tail.) If it's pointed it's a male, if it's rounded it's a female.