Knowledge Capsules | Detecting unused CSS selectors
http://www.aggiorno.com/blogs/aggiornings/post/Detecting-unused-CSS-selectors-.aspx
geuis's helium-css at master - GitHub
http://github.com/geuis/helium-css

Helium is a tool for discovering unused CSS across many pages on a web site. The tool is javascript-based and runs from the browser.

Helium - javascript tool to scan your site and show unused CSS

integrate this into release procedure

determine unused css

tool for identifying unused css