How do I know my fry are eating?

I have 6 fry in my main tank. They are platies and they were born last sabbath [by that I mean last week's saturday]. Im not sure if they've been eating. I can see about a few eat a bit, and I can make out poo coming out of a few of them. How do I know if they're all eating or are eating enough? I cant watch them 24/7. They're all swimming fine. They're in the tank with the bigger fish but are seperated in a makeshift fry home [plastic bottle with holes for water flow].