Finding "unclosed element"

How do you find unclosed elements.  I'm trying to clean up code, but when I look at the source in Firefox,  the close body tag reports the following error:
End tag for "body" seen, but there were unclosed elements

This occurs on even simple pages.  I understand what the message means but how do I go about finding the "unclosed element"?  Is there an easier way to look at the source to find this error?