[bumfluff]

I would ask them to give a critique on a piece of work without preparation. A lot of portfolio work has input from other people, having a surprise crit can be a good way of evaluating their eye for design and detail.

I'd also be tempted to ask them for examples of jobs they did that they felt were successful and why, but focussing on the team process rather than the overall look (if they need to be working as part of a team).