This tip is courtesy of Daniel Stine, author of several Revit books (listed in previous post). If you are familiar with the Grid Generator extension for Revit you might have already seen this? If not...have a look!
This extension provides a nice summary of all the levels in your project AND their elevations. Also nice is that you don't really have to do anything, just launch the Grid Generator tool and then make sure the Levels option is selected.
It isn't always easy to see them all when some have been hidden or their extents are affecting what you see in sections and elevations. Thanks Daniel!