Secure Key exchange

I have at this point signed keys of 6 people (the first three over dinner at a chinese restaurant -- this didn't start a trend, unfortunately :-) I haven't signed John Gilmore's key (even though I work for him) since I haven't actually seen him in person, though I may get a chance to when I'm in California next week -- this will create a link between east-coast and west-coast signatures, though possibly not the first. If you meet someone claiming to be John Gilmore, how will you know he's not an impostor?