First, Stealth Agents:
My agent was/is a stealth agent. She was/is an A-lister in hiding. You will run across these in your search. Oh, God, you say. That's a whole new kettle of fish. Real short: in most cases, a stealth agent does not have a website, Twitter feed, FB account--NONE of it. No footprint. They might only be registered in a few online media listings, like Agent Query, and there you will find almost nothing about them except a phone number and an email address. They might even announce that they are closed to queries at the present. I Googled one such agent and found a few listings associated with books she had repped on Amazon.
When I Googled those authors I found them to be best-sellers and name celebrities. It was quite a shock to find out how successful she was.
I also found more information about her at my AW writing site which confirmed my suspicions. Everyone agreed that she WAS a very successful agent and had dinosaur longevity. She only stays/stayed invisible because her stable was full, she wasn't taking on queries and she concentrated on furthering the careers of her current authors. Quite noble, actually. The secret handshake to her was, simply sending her a proposal whether she liked it or not! Hah! I did this and she took me on, bowled over with the number of books I had finished and ready. She said, "Oh, you know the way to my heart-you came and got me." I don't recommend this strategy for other authors. It's a coin flip. Don’t try to get into the Richard Curtiss Agency this way—he’ll flatly tell you he’s only interested in non-fiction. Yet, he has his ear open to the right scout and movie exec.
The A-lister: look up your favorite genre or contemporary books and authors at the major book stores and look in the dedication pages. Make note if the publisher is a large one, i.e S&S, Ace, Harlequin, Penguin/Random, etc. You'll find out what agent sold those books. Large, popular houses = A-list agents (for the most part). It's a good feeler gauge, but some new agents make big sales too, just not that many of them. Google the name of the agent on the Amazon book page and, wallah! You'll see how many books, ranks, best sellers the agent has dealt with. Better: ask the agent straight away for their rep history, author names she reps, length of business and so on. An A-list agent is How Big, How Many and for How Long. You’re likely to get an agent/author contract.
B-list agents, who also sometimes call themselves "boutique (or medium-sized) agencies" generally have several dozen sales under their belts with medium sized publishers and maybe a few large ones. Heh, don't get me started on Medium size, that's a whole new article. B-list agents are oft times known by first names in the industry and their over-the-line phone pitches are not frowned upon. Many of them (not all) are based in the NY area and have kissy-face lunches with publishers, as well as the A-listers. Agent/author contracts are common.
C-listers are new, have only a couple of sales and are eager to pull in new authors. I say, "pull" because they can compete and be aggressive about signing you. They're just starting out. I can’t say that I recommend them. Caveat: it can also be said that a C-list agent will pull out all stops to find you a publisher and that can mean dozens and dozens of rounds. I have heard of a few authors who are repped by C-list agents and are happy with the arrangement and have no complaints. But I beg them to wait two or three years and tell me the same thing. A no-name agent in the business can be as bad as no agent at all. Besides, do you want your book tied up for two or three years indefinitely? They are more than likely to sign an author to more than one book. You might get an author/agent contract.
Is there such a thing as a D-list agent? Well, yeah. They are the ones that sign you for a one-book contract (having no care about your career), charge a reading fee for a synopses, chapters or full and exceed the royalty share of 15%. They also tack on undefined administrative costs—postage—copying services, mailers and sometimes an hourly rate. They most likely will offer you their lofty editorial services, or refer you which gets them a kickback. You might not get a written contract with them, but it’s not likely.
ETA: Don’t fret if you don’t get a author/agent contract with any agent. It’s no reason to panic and it happens all the time. In this business, your/their word does carry a bond. No so in the movie industry.
Hope this helps a little.