Well, I'm certainly not an expert, and I guess I had a lot of luck, but I've been able to tell my female platy was pregnant
She wasn't really that fat, but when looking at her from the front, you could notice it was a bit more swollen than normal, hard to explain, but I guess you would see it immediately...
So I put her in a breeder net, and indeed, 3 days later, she had the baby platies
normally I would have let her stay in the main tank, but I really wanted platy babies, soooooooooo cute!