Drew “All in the Head” McLellan has made an awesome bookmarklet called rel-lint that detects XFN, rel-tag and other microformats:
This is a lint tool (like a non-authoritative validator) for XFN, rel-tag and other microformats that make use of the rel attribute of links. It takes the form of a bookmarklet you can add to your browser and then run against any page. rel-lint will check for known XFN values, flag values it doesn’t recognise (even though they may well be correct) and display the tag values for rel-tagged links.
Factory Joe 