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.
Add this to your bookmark bar: rel-lint
This bookmarklet uses JavaScript to add a link to an external JavaScript file to the frontmost page. Any time you allow a script to do this, you should examine the source and satisfy yourself that nothing malicious (such as cookie theft) is occurring. This particular bookmarklet does nothing evil, but the bad guys will say that too. You need to check and be sure yourself.