I use Firebug and IE Developer Toolbar all the time to debug tricky CSS problems. But occasionally, a tricky bug comes up that only appears when you go to print the page.
What techniques/tools do you use to diagnose problems like this? Is there a way to get more use out of traditional CSS debugging tools in print view?
Updated: I already use a PDF printer to avoid wasting paper; my problem is that I can't right-click on the printed DOM. Some of the other answers below are quite helpful, thanks. :-)