Is my balloon molly actually pregnant and how can I identify how far along she is?

I bought the molly a couple days ago and its stomach looked a little large than others in the store's tank. I know it's a female for sure, but I can't tell if it's actually pregnant. I know that you can see black spots in the gravid area when they are pregnant, but my balloon molly is dalmatian colored so I that process doesn't work. Any other ideas on how I might tell if she is pregnant other than just watching her get larger and give birth? Also, if she is, can I tell how far along she is, because I don't want to lose any fry or keep her in the breeding chamber too long. Thanks!