My most valuable two questions have always been "What part of the stack you're working with do you understand best? Can you explain to me how it works?" and "Wh

My most valuable two questions have always been "What part of the stack you're working with do you understand best? Can you explain to me how it works?" and "What is the most complex modification/extension of the stack you've ever done? Can you explain to me how you did it?" That will show you exactly how well they know their tools, how and how far they bent them to their will, how passionate they are about them and how well they can communicate on difficult technical topics. I'd be hard-pressed to find other questions that consistently give me that much information about a candidate.