It's not the most intuitive, and not really that well documented. As another commenter mentioned, Firebug is a must-have for this process. I also find Safari's Web Inspector, and IE's Developer Toolbar useful (IE, of course, goes by it's own rules). Each of them will let you isolate each tag or container, will show you what css rules are affecting that piece, and which CSS file the rule is in, as well as their order of priority (top to bottom).
Generally speaking, a I think a YUI menubar will involve styles from YUI Overlay, YUI Menu, YUI Menubar, and then any global styles (or resets) that you're using.
> I need to style my menu according to my clients design. I am looking for a list of styles that i will redefine for the sam skin and have not been able to find that on the yui developer site,
> is that list available?