The balloon mollies are harder to breed than most because of their shape - but being livebearers I think you can trust the females to get pregnant if there is a male within 100km. I'm sure there is a gang of aliens that walks around with a bag of male fish making sure all the females get pregnant! lol - overrun with fry and spending most of my pocket money on fish food.
The 'gravid spot' isn't really obvious in anything except guppies and sometimes swordtails and platies. Not in mollies in my experience anyway. The best way to tell is when the female's stomach swells up. When she is obviously a few days from giving birth some people will put them into a breeding net or move them to a seperate tank to ensure other fish don't harass her or eat the fry. Most of mine drop in the tank and I rescue what I can because I'm a sentimental twit and I don't seem to realise that I have more than 130 BABY PLATIES!