An interview with a potential copywriter is an audition. Don't be afraid to give them a writing assignment, and give them a couple days to complete it. That will give them time to research your topic and give you an opportunity to see how well they grasp your industry and, more importantly, their technical skills and style. Is their submission riddled with industry speak and jargon, poor grammar, or bad flow from one thought to the next? Their writing assignment may reveal more about him or her than a portfolio full of previously written works.