Ok, well if your expecting fry the female swordtail should have a large belly and a gravid spot (sometimes a gravid spot is hard to see with swordtail but you can tell the fry are still there). When the female is really to give birth see will be looking for a private spot around your tank, mostly near or behind the heater.
You you have a spare tank or floating fry tank? These will be usful if you want to save most of the fry. All you gotta do is catch the fry and place them in the other tank or floating fr tank. The other tank should not have any fish that would be able to eat them.
When feeding swordtail fry, flake food crushed up into small pecies are good or finley chopped up bloodworms. Baby Brine shrimp is small enough for them to eat too.
Hope this helps ^__^