The viewer (AutoVue from Cimmetry) that comes with Adept has a great 'Compare' function where you can load a couple drawings and, well, Compare them against each other. The viewer will highlight and show only the parts of the drawings that are unique. We don't use it all THAT often, but man, when you DO need it, it sure is nice to have.
In AutoCAD, I usually just open the files I want to compare and hold down CTRL+Tab to cycle through them and that helps, but on a really busy drawing, that doesn't quite cut it.