That's a balloon molly. Just has a huge belly
Look at the Anal fin ( the fin closest to the anus) and compare it to your gold colored Molly. If it is tubular like the Gold's ( and the Guppy in the picture), it's a male. If it's more triangular ( like an upside down sail from a sailboat) it's a female. Unfortunately, the picture you posted does not show this fin on the white fish.
Yep thats the way you do it. You can kinda tell in the first pic that its a male