I can't seem to find the post, I know it exists, but it's to check the foaf string on a journal. Just right click on someone's profile page and select 'view page source'
If you scroll down a bit in the code, you see something that says 'name="foaf:maker" content="foaf:' followed by a bunch of numbers. The numbers corresponding to the e-mail address associated with the journal in question. If the numbers to two journals are the same, they are registered under the same e-mail address.
It's not an exact science since people can always make fake e-mail addresses or use multiples, but it does give someone a basis to work off of for avoiding creepers (so cyberstalking awesome people).