phranque - 11:14 pm on Jun 12, 2013 (gmt 0)
google page speed used to report unused css on a page-by-page basis.
it looks like they dropped that feature but perhaps if you have an older version of the plugin.
the chrome browser has this feature in the development tools.
as far as reducing the "weight" of css you should also consider minifying the code, using gzip compression where acceptable, and configuring your server to provide the proper HTTP Response headers to support browser and proxy caching properly.