You can't really be sure without running all of your javascript code and checking all of your pages, after all some hidden bit of javascript might change a style based on some event to match one in the css, it might even do it by combining various terms to form the selector, and it might manipulate the ids and classes of elements already on the page.
That's a tricky problem. I'm not aware of a tool, but if you find one I'll be happy to test it, I have a few of those questionable items in my style sheets that are still there 'just in case'.
