The difficulty you'll have with this is that the vast majority of stuff on the "C" drive doesn't have to be there. The only bits that do are some critical system files, but applications, app data, downloads, documents, media, etc etc etc, can all be moved to another drive without incident. The question you need to answer for yourself is what of those you're looking for, and what you're hoping to achieve by it.

I second what Art said. It might be better to ask what files must be on C, rather than asking about a program to detect ones that don't.