How many homes (of your type) have they listed in the last 3 / 6 / 12 months. Of those homes, how many have sold / expired / cancelled.
Of the homes that have sold... what was their list-to-sale ratio (from the original listing price) by percentage, and how many days on market did they take.
That should give you a snapshot of the type of business they produce. And then ask them to tell you why you should hire them over anybody else. When they answer, sit back and listen... let them talk (we love to talk about ourselves).
Do that with at least 3 agents... if you haven't found someone you "click" with in that time... interview 3 more.