This could be one way to do it, change the css to media="print" and do a print... you can change it on a page with firebug and see the difference it makes on print...
That's what Print Preview should be for; I'm not sure how Firefox and other browsers handle this on other platforms, but OS X builds the Preview right into the Print dialog, so I don't see the Print Preview option. That would be the easiest way to view how a page looks when printed.
Even on print preview, if the css sheet does not include print (like media="screen") it will not be pulled in, at least in firefox (on linux that I can test). If you open firebug and set the css sheets to media="all" it will pull the css into the print preview and look (more) correct
