PLaty fry are really good at hiding...unlike guppy fry that seem happy to swim around and get eaten

I had a dozen in my tank for 24 hours and didn't even know the mother had given birth

First I saw was a pair of eyes peeping from behind a leaf...lol

So I wouldn't worry about them..they will do just fine until she finishes.
Platies breed better when just left to it in a community tank, unless you're wanting to breed a certain strain of them..then its a matter of pairing up. But most females when you buy them will be pregnant, as they tend to have already mated before they are seperated into sex groups at the breeder.
Knowing that they have mated...ermmmm well most of the time you won't..lol but from my experience they will mate a few days after giving birth, hense the quick turn around of batches of fry. They can on occasion be seen *courting*..lol
Preggers... well its easier to see on some fish than others..the lighter coloured fish is easy. A dark area apears at the rear of the fish, just above the poop hole (known as the gravid spot) This will grow as the babies grow and its about 21/28 days to giving birth. Guppies are much easier. Its easy to tell the sexes apart and easier to know if the fish is preggers as the gravid spot is very visable on all colours. Also easier to work out when they are going to give birth as they get large square bellys just before.