Common livebearers are easy to sex once they are mature. The whole thing is based on the shape of their anal fins, the fins right next to where they eliminate their waste. Swordtails, guppies, platies, mollies and limias all have the same basic shape for a male and the same basic shape for a female. This is a picture of a male Heterandria formosa that also has the same fin shape for a male.The anal fin is long and thin and is correctly called a gonopodium.
This is a female of the same species with the typical triangular anal fin.
The pictures are a bit grainy because they have been blown up to show the fish. The actual fish are only about 2 cm long when full grown.
There is another less common group of fish called goodeids that are also livebearers and are sexed using anal fin shape. The generic name for goodeids is splitfins so if you see a fish with splitfin in its common name it is probably a goodeid.
The anal fin on this male Xenotoca eiseni is showing the typical split in the anal fin with the front part of the fin shorter than the back part.
The female Xenotoca eiseni in this picture shows the typical single triangular fin of a goodeid.
With good lighting, and being pulled back to show more than the fish, a male Xenotoca can look more like this. I love the colors they have when they are feeling safe in the tank. Unfortunately, when pulled back, the fin shapes are hard to pick out.