My guess would be through some sort of computer club / group. I got to meet Ken Thompson because he was visiting the university I worked at and the unix users group on campus convinced him to stop by. Join up these sorts of groups, get on mailing lists, ask around.

Luck, mainly. I'm taking an AI class at Stanford, and it turns out that the professor knows McCarthy. The class is philosophical, not technical, so I don't know how all of these technical questions are going to be received, but I can't not ask them.

A philosophical AI class? Please elaborate. I took an AI class for my CS degree and it was technical.

There are technical AI classes here that are required for a CS major. This one is an elective called "Can Machines Think? Can Machines Feel?". We read famous AI papers (including McCarthy's "Ascribing Mental Qualities to Machines") and talk about what's possible and what's not.