First of all, sexing livebearers is easy Female livebearers have a normal, fan shaped anal fin; that's the one on the fish's 'bottom', just in front of the tail.
Males, once they are mature, have that fin modified into a rod like structure called a gonopodium, that the male fish uses like a penis.
You need to know how to sex your fish so you don't end up with too many males. Male livebearers are, I'm afraid to say, sex mad, and if you have more males than females they can, literally, harass the females to death.
As for signs of pregnancy; well, in one way it doesn't really matter, as the mother fish and fry are better off in the main tank, rather than being put in a breeding box or net (they're usually too small for the female to feel comfortable, and too small for the fry to grow properly), but the female will get larger and larger until, in the last day or two, she'll do what's called 'squaring off', where her belly will look very square, like she's swallowed a box. Once you see that, you can guess the female will soon give birth.
The fry don't need any special care; just lots of good, clean water and plenty of hiding places so the parents don't eat them.