If you feel like you click and they understand your vision or have suggestions that you can visualize, I think that is a good start. You want someone who you get along with, and who seems to have a genuine interest in you and your wedding. When you initially contacted them, how long did it take them to get back to you? This initial interaction can set the tone for what their style is so pay close attention :)
*Be sure you 'click'- you must be able to work together and trust your coordinator implicitly
*Be sure your potential coordinator truly listens to you and everything you want.
*"Rightful Confidence"- your coordinator candidates must know their job well and exude confidence. Whenever something adverse occurs, your planner must be able to take care of it without skipping a beat.
As a wedding planner, it is so important that you mesh well with the planner. Remember, this person is going to be you side kick for however long you hire hm or her and you need to be able to work with this person as well as communicate with them. Always look at their past work, or their ideas, if they match your ideas for YOUR big day, then that is a good thing. The last thing you need is a bossy planner that doesnt listen to you.
Most of all it should be someone you can feel at ease with. Do your conversations end with once sentence or do you find yourself meeting with them initially for over an hour talking about the event ideas in general (not just answering questions). Look not only for their verbal cues on if they are confident, interested, and passionate about your event, but also at their non verbal cues. Much like the dress, you'll probably just KNOW when you've found your match!