Livebearers, like platies, mollies, guppies and swordtails, are easy to sex.
You need to look at the anal fin; that's the one on the fish's bottom, just in front of the tail. In females it's a normal, fan shaped fin. In males, that fin is modified into a rod-like structure called a gonopodium, which the fish uses like a penis.
Most female livebearers are pregnant when you get them, as shops rarely separate them early enough. They can also store 'packets' of sperm, so can continue getting pregnant even if there are no males in the tank with them.