I came across this firefox addon which did a good job for me. You just have to run the addon, and save the file where no unused css is present.
1) Download the addon at this page: https://addons.mozilla.org/en-us/firefox/addon/css-usage/
2) Run the scan and save your css file like shown in this printscreen:
3) Be aware the addon scans the CURRENT page, you might have some “unused css” on this page but it’s needed on other pages.
For a wordpress site for example, you might not use some css on the index.php page but you can use it on the single.php page.
4) in your saved file, you will see the UNUSED text before an unused css code, you may want to remove that code in order to clean up your css.
Update: a solution for the multiple page problem is using the dust me selector add-on and check your site with the sitemap spider !
Hope this helps