I found this on the Literary Rambles blog–it’s a long post full of good advice on finding a good agent match. Here’s the conclusion:
Focus on legitimacy. Focus on sales and clients. Take all the time you might spend assessing details and create a really rockin’ list of questions to ask an agent if they offer. Because that’s where it’s at. THAT is where you can get the low-down, from the source, and make a truly informed decision. You’re not bound to accept an offer when you query an agent. Use that to your advantage. Query all the agents that MIGHT be a good match and then slow down, interview them, and find out for sure.