I generally find that asking specific questions about a technology or technique is not really valuable. My preferred method of interviewing is to sit down and talk with the candidate about "stuff", which should include said technologies/techniques. After a fashion you will get a good understanding of what they know about the subject; where they are strong and where they are just blowing smoke. You'll also know more about how they would integrate into your team.<p>So just relax and talk to the candidate and don't worry about asking the magic questions that will answer all your prayers. :)
I tend to ask, not how you use the framework, but rather how the framework works. How does routing actually work? How do named scopes work? How would you implement the {before|after} filters on models... etc.<p>It's actually somewhat more difficult to tell whether someone knows how to use Rails well. My only suggestion is to sit down in front of an editor with them and work through actually building something small but meaningful.
I might ask the interviewee about stuff from the future of rails. If answered correctly, it shows how someone can be in a continuous learning mode and eager to go the extra mile.
I'd ask the candidate to pair program with a developer for a week. You'll know immediately if the candidate can integrate with the team. Plus it'll reveal his knowledge during the session.
Ask them to develop something that involves caching the paginated search results of a double-nested resource with file attachments in as little time as possible. hehe