Also ask about the broker's deal history, and current workload. If the broker typically does large deals (20,000 SF+) and is currently working two of those large deals at the moment, a small 3,000 SF deal could be low on his priority list to attend to your needs. Sometimes its a good thing to get a young, hungry broker to work your deal because he/she might pay more attention to you than a "seasoned expert" who only does large transactions. Keep in mind, this is not true for everyone. Just something to think about.

I also agree with adb1's comment that you should interview multiple brokers. You will definitely be able to figure out a good personality fit if you do this.

You should look for a few things:
1. make sure they have at least 3 or 4 years of experience working in your market.
2. if you're needs involve a specific type of space (e.g. retail space) make sure the broker specifically focuses on that type of space.
3. get at least 3 references from previous clients
4. ask specific questions about market rates, vacancy and recent comps to get a sense of their knowledge.
5. Take the time to interview a few brokers. This is an important decision.