You can assume that all female livebearers are pregnant, all the time.
It can be very hard to tell if balloon mollies are pregnant, because of their shape. you just have to wait and see!
With guppies and normal shaped livebearers, you want to look out for them 'squaring off'; their stomach will develop distinct corners, as if she's swallowed a box. Once that happens they usually give birth within a day or so.
We never recommend the use of breeding boxes. All female livebearers are better off left in the main tank, unless you have a properly heated and filtered spare tank, of at least 10 gallons. Add plenty of live plants to the main tank, if you want some fry to survive.