How can I apply an "active" state to a 2nd level menu bar

Can anyone give me specific instructions/example of how to add an "active" class to a menubar.

I'm working on a reusable snippet for 2nd level menubar (not the top level navigation). I thought the menu bars might apply an active class by default, but this does not seem to be the case.

I'd like it to be reusable, so that if the page title matches the menu item, it will automatically apply the active class. Is this possible? What is the best way to accomplish this?