sexing livebearers
i'm sure they meant how do you sex the fish, not how do you mate them. they know how to mate on their own lol.
yes, there is a way to tell, very easily. look at the anal fin (which is right next to where the poop comes out).
in females, the anal fin is triangular and looks like a normal fin. in males, though, the anal fin is modified into a penis-like structure called a gonopodium. under normal circumstances the fin points backwards and looks kind of like a normal fin folded up. but if the male is chasing after females you might see it swing forward trying to copulate. it's actually quite easy to tell, easier to see than describe!
ps. if you're still having trouble, go to the lfs and look at swordtails. male swordtails have swords - look at their anal fin and you'll see a gonopodium. compare it to the swordless females
mollies, platties, guppies, and swordtails all have the same fin adaptation. hope this helps!