In Google Chrome, open up the page you want find the unused css rules in.
On the menu select View -> Developer -> Developer Tools
Select the Audits tab in the toolbar.
Make sure the Web Page Performance is checked, then Run for Audit Present State.
You'll have a list of unused CSS rules.
Hope that helps.
I've been searching and i can't find any ff addons or javascript for finding unused css in ajax apps.
dust-me selectors can do a site-crawl, but i'm looking for something that examines loaded-in content...
I'd like something where i can press 'record' and then make a load of clicks which will check off the used selectors, and hoping to find an existing one rather than try to write my own with jquery!