I would stray away from an on-the-spot design task. Design and UI is a very research and strategy oriented task. The result of an hour or day long test may no

I would stray away from an on-the-spot design task. Design and UI is a very research and strategy oriented task. The result of an hour or day long test may not be an accurate representation of what the designer is capable of. If you like his work, commission him for a freelance project as the test. Give it milestones and specs like any other project. Based on the results - concerning time management, style, ability to digest the specs, etc, you can have a better idea of the applicant.