better to look at his portfolio then to put him under pressure to complete a test in front of you, which doesn't reflect real-world work environment. More important that you ask him about stuff, see if he is smart or stupid - - does he know whats going on in the world? Have opinions on stuff? Seem intelligent to you? Also if he seems annoying. I'd spend all my interview with that sorta stuff, relaxed conversation, and look at portfolio, then ask him to do a small mockup from a PSD template. Pay him for it or see if he does it for free. If it comes back on time (give a deadline) and done well you know he has the skills, isnt a moron, and wont drive you crazy
