In inspection mode, Firebug allows you to identify specific page (DOM) elements and get a listing of their class or id names and actual CSS code. This inspection tool is a great aid to finding specific CSS elements for making styling and modification changes.
Firebug has two major implementations: an extension (add-on) for Mozilla Firefox and a bookmarklet implementation called Firebug Lite. A cross-browser version is in development.
There are similar, but less capable tools, for other browsers.