When my mom was buying her new house, it was a once in a lifetime thing after selling off the house she'd lived in for 40 years. So she kind of just learned the hard way as she went. Some of the tips I can pass onto you are:

- Go out with every real estate that you can find. It can't hurt to try once. And let them suggest places that you might have been adverse to buying a house in. It can't hurt to just look. (Unless it's an unsafe neighborhood, that's a different story.)

- Ask a bazillion questions. Even if they seem like dumb questions. See how they treat you... do they treat you courteously, or talk down to you like you're stupid? Do they take your questions seriously? Do they look things up to give you thoughtful, truthful answers? Try to get an idea of who cares about what you want to know. If they treat you well when you ask small questions, hopefully they'll treat you just as well later when you have big, serious questions.

- Request information like school districts, flood zones, evacuation routes, etc. See who says that's not their job and who goes the extra mile to provide them for you.

- If applicable, research their answers. See if anyone lied.

- Get an idea of what you want and tell them. See how well they stick to it. When my mom was house hunting, she said she didn't want a pool. Some pushed the pool houses on her, some respected her request and showed her no-pool houses only, some showed no-pool houses only but gave info about pool houses on the side saying "I know you want no-pool houses but just in case, these are here to view if you decide you'd like to." That weeded some agents out right there.

Basically, give everyone a test drive, and see who looks like they're the most interested in your best interest instead of their own. Take a friend or family member with you if you can, for a second opinion. And good luck. :)

I always go by personal recommendations. You can also go through major, trusted realty groups and organizations.

This seems to be a pretty good list of questions:
http://homebuying.about.com/od/realestateagents/tp/Agentinterview.htm

Some of the important things from that list:

How Long Have You Been in the Business?

What is Your Average List-Price-to-Sales-Price Ratio?

What is Your Best Marketing Plan or Strategy for My Needs?

References

And of course, How Much Do You Charge?

More info in the source.

If you really want a good one, you need to do some work. Get a list and suggestions from friends. Then ask for a list of clients she's worked with who you can speak with. Finally ask them the questions that are important to you. It will vary wildly from person to person. In my case we wanted someone who was patient. We wanted to look at A LOT of houses. We also had a pretty wild budget. (We were not sure if we wanted to go cheap and fix up or buy turnkey.)

Hope this helps. Good Luck in your search!