Job fit should be obvious from their portfolio, so I would focus on questions that would help shortlist designers for their culture fit.
Culture fit is way more important to the long term success of both the designers and the company. To do this, though, you need to get them talking about past experiences. See how they describe previous jobs and managers. Do they take ownership of failures, or is it always due to someone else? Do they have a history of being unhappy, or is every experience a positive one?
Answer is probably a bit off topic, so sorry about that. But we have four separate interviews for our designers, and only one focuses on abilities, and that is more just to ensure that what they say they did in their portfolio is actually their own work.
The other three interviews are focusing on how they fit our core values. I have done this three times in the last year, and I haven't regretted my hires yet.