All you can do is what Karen suggested... You can also try putting ads on Craigslist about the found cat as well as looking through craigslist ads for missing cats.

Keep in mind, though, that if you are feeding the cat, in a majority of states after a certain amount of time that means you are legally the owner (thanks Animal Cops for teaching me that!) so if you want to adopt her, great; if not, then I would recommend looking for a new home for her.