Your questions will differ from what you would ask a buyer's agent, so don't apply all of these guidelines.
1. Referrals - testimonials. Look online or ask the realtor to speak to former clients. The more in depth the overall testimonials, the more passionate the agent is likely to be. A number of referrals that say things like, "great, really good job, highly recommend" are a concern.
2. Personality: But be careful about the extension of self thing. You want somebody you can get along with, and who treats you, and responds to you, in a manner you feel comfortable with. But this is a business, not a dating service, so try not to get too wrapped up in the minutia of the interview.
3. Sales history - see how long it took to sell various listings and at what percentage of list price, but don't fixate on one as there are sometimes variables beyond a realtor's control (a terrible property, a reluctant homeseller). An agent must, absolutely must know how to price your property or you will regret your choice in realtor and end up back on Trulia's advice purgatory.
4. Marketing - Since the majority of homes sell through websites affiliated with Realtor owned MLS, don't get too caught up in Open Houses (scam to get more clients for realtor), Promotions, seldom used classifieds, man in a chicken costume recruiting passers by, etc.
But do ask to see past marketing materials. A buyer leaving a house with a detailed list of upgrades and photos is more likely to remember it after seeing 10 other properties. This should look professional, not photo-shopped by an 8 year old. Floor plans are great ideas as are drawings showing possible improvements that can be done (even including estimated cost). Photos and videos posted online should be high quality (always ask to see previous listing materials).
4 b. Staging - A agent must understand what needs to go and what can stay and what should be brought in. If you are told to simply vacuum and clean up, you are stuck with a Realtor Reject.
Homes sell because they show well and are priced right. If an agent can prove they have this ability, you found your realtor.