Good suggestions, all. You can also check with the Humane Society or pound --
we keep a "lost & found" database at ours. IMO, if somebody doesn't care
enough
about the cat to check there, they don't deserve the cat back.

Another thing I did was call the vets in the area to see if a cat matching her
description (we found an adult white cat) had been reported lost, and left a
contact number if someone was looking for her. I also checked all the lost pet
ads in neighborhood and city papers, but we didn't run any found ads. I did
register her as found with the Humane Society, and I took her in to a vet to be
scanned for a microchip. We also asked the neighborhood kids if they'd ever
seen this cat before. But IMO, you can only do so much. If the person who lost
the cat isn't making flyers and running ads and calling the shelters, I'd
assume they don't care that much about finding her/him. In our case, the cat
was so emaciated either someone had lost her quite a while ago, she was dumped,
or whoever had her was neglecting her anyway.