Good advice from Aaron Swartz:
To find out whether someone’s smart, I just have a casual conversation with them. I do everything I can to take off any pressure off: I meet at a cafe, I make it clear it’s not an interview, I do my best to be casual and friendly. Under no circumstances do I ask them any standard "interview questions" — I just chat with them like I would with someone I met at a party. (If you ask people at parties to name their greatest strengths and weaknesses or to estimate the number of piano tuners in Chicago, you’ve got bigger problems.) I think it’s pretty easy to tell whether someone’s smart in casual conversation. I constantly make judgments about whether people I meet are smart, just like I constantly make judgments about whether people I see are attractive.
It’s been my experience as well that you can tell more about a person’s innate capabilities, intelligence and curiosity through casual conversation and meandering topics than a structured, prepared interview.
(via DF)