Do you know anyone who has bought or sold a home in the last couple of years? Your best bet is always first-hand recommendations if you can get them.
Aside from that, visiting open houses on the weekend in your target neighborhoods will give you the chance to casually talk to various agents to see how you mesh with them. I find you can learn a lot about an agent even in a brief interaction.
I have an established relationship with an agent. Even so, I like going to open houses just for fun and I like to study how others approach it. I find that good agents do the following:
1) They actually make an effort to speak to people. I know this seems basic but I can promise you many of them won't do more than say hello if that. They should, after some small talk, ask if you have an agent and for your contact information. Many of them seem afraid to do even that. This tells you a great deal about how aggressive and assertive they are.
2) They should be open to meeting with you again to answer any questions you have in detail to determine if you're a good match.
3) They treat everyone equally/not assume your socioeconomic status by your appearance. Dress casually and drive the least nice car you have. See which agents are still friendly and engaging. The good ones know you can't judge someone's buying power by their choice of car and clothes.
4) They should be a full-time agent (i.e. not working some other job), willing and able to meet and communicate with you on YOUR schedule.