To add on to the "Tell me about a project you are most proud of?" question, having gone through different stages from burnout to re-kindling of the hacker within, I can see it measures attitude well, which is crucial. For aptitude, however I would go a few layers deeper. "What was the toughest problem you have worked on or solved?", "How did you solve it?", "What did you learn?" the choices of problems tell you as much if not more than the answers about where the candidate's passion stands. Are they vague or specific? Do they use industry terms or are they self taught? What types of issues make their eyes spark? etc. If you are hiring entry level people, passion is most important, but if you need to have people who hit the ground running in an area, nothing beats a nice rigorous interview, with some code writing involved, in addition to your question.