Do a water change as often as it is needed to keep your levels below 0.25 ppm of ammonia and below 0.25 ppm of nitrites. The fry, if any survive, will also benefit from good water conditions. I have seen some half truths about livebearers always carrying enough sperm to support 6 months of fry production. It is likely to be true for some of the more common livebearers but is not true for things like goodeids. They need a fresh mating for each brood to be produced.
To tell a male from a female in platies and many of the more common livebearers, The fin right behind where they eliminate waste will be a triangular shape on a female and will be modified into a long thin thing that is used to impregnate the female on a male. It is called a gonopodium and I am sure you can find pictures of one using that word in a google search. Many of the common fish also have secondary characteristics such as colored fins, a sword shaped tail etc. but the gonopodium is the sure way to tell on a common livebearer.