To evaluate an agent, look for Credibility and Personality. Credibility is proven sales of books like yours. Any real agency will be glad to tell you which books they've sold, but probably won't give you the phone numbers of their authors. Read the acknowledgments section of the books they mention if you're still skeptical. Your allegiance as an author will likely be to your agent, not your publisher.
Agents invest in authors' careers.
We want to sell your first book and your tenth. Find someone whose personality meshes with yours - whatever that means to you. Once you've got an agent, a signed contract, and your hopes up, don't quit your day job. It can take a while for the best of agents to sell a book, but as long as you're getting rejection slips, you'll know that at least it's out there circulating. Contact an agent by the method they prefer, as stated most often in the directory in which you found their name.