I'll skip the code part - I guess you know how to evaluate that one.
I'm not a great interviewer myself, but what I learned from watching others is: if you ask open ended questions and encourage longer answers you can try to find out what level the dev is thinking at. An extreme version would be something like "tell me about starting a new web project in Ruby". Does that person concentrate on "rails new" creating an empty app? On setting the environment of source control, place for collaboration? On gathering requirements to get an idea what framework/libraries would be needed?
Don't expect a single question to reveal everything, but a few can give you an idea what level to expect.
