My cat was pretty feral when I found her and would NEVER purr as far as I could tell. As she got older and began to trust me, the purring got louder and now at 7 months old her purr is audible from 3 feet away! Put a finger on the kitten's throat the next time you are about to provide a meal.. that's a sure way to find out.
I almost forgot.. she was 6 weeks old when I got her!