lucy24 - 12:31 am on Jun 13, 2013 (gmt 0)
There are plenty of tools that check for unused CSS within a single document. A site-wide check is tricky-- in both directions. ("Oh, ###, all this time it's been using my browser's generic <li> style when I could swear I had a ream of stuff that was specific to 'ul.options li'".)
But, hey, as long as you've got the file open, why not run it through the CSS validator? If it yaps at something whose name you recognize, that's when it's cleanup time.