I am by no means an expert on how to select agents to query. I’m just this writer with an agent and an internship, you know? But I think I collected a solid list of criteria over my querying days, so I’m going to share it. (I didn’t actually have a list I’d consult—over time, I just learned how to look for these things without really realizing I was looking for them.) Anyway, yeah, this is not exhaustive, nor should it be your only resource.

With that said, here’s how I did it. I’m not you, so your priorities may be different—keep that in mind! Also, all these resources listed are free. I don’t really recommend paying for a yearly QueryTracker subscription—I spent $0.00 on that site, used it daily, and learned all I needed to know—and Publishers Marketplace, while content-rich, isn’t worth the money if you don’t work on the other side of the desk.

1. Find out about an agent. This could be on Twitter; Absolute Write’s Bewares, Recommendations, and Background Checks forum (I typically read the recently updated threads a few times a week, even and especially now); QueryTracker (regularly updated with new/new-to-QT agent listings on the bottom of the home page); AgentQuery; or through word of mouth, from critique partners or writer-friends, etc. There’s no best source on which to discover agents; they all have pros and cons and quirks—querying writers don’t usually air complaints with specific, named agents publicly on Twitter since their real names are attached; AW has the opposite problem, wherein almost every agent with an industry presence to speak of has an anonymous complaint lodged against them somewhere; QT has well over a thousand agents in its database, which can make it difficult to cull your list; AgentQuery is a bit of a hassle to navigate. That said, they can all be invaluable resources.
2. Find the agency website, whether from one of those initial resources or via Google. Take general note of the site—is it professional? Relatively easy to navigate? Are the agents’ names given?—but don’t put too much stock in this. ICM, one of the biggest and most respected literary agencies, gives you barely any information whatsoever on their site. Frankly, they don’t have to. They’re ICM. Compare to The Bent Agency (which I am un-coincidentally represented by). TBA’s site gives detailed bios for each agent, a list of clients, a broad overview of what they do for their authors, recent agency updates, contact information, and other goodies. TBA and ICM are both perfectly respectable agencies with great sales and solid industry reputations. So what’s the difference here? You can relatively safely speculate that TBA agents are more actively building their lists (industry-speak for “signing new authors”) than ICM agents. This isn’t the case for every agency ever, and honestly, I’m taking an educated guess with that conclusion. But in a solid amount of cases, agents who are eager to sign new clients won’t make that process difficult for those potential clients.
3. Get the agent’s bio, somehow. Occasionally I used Writer’s Digest as a last resort for this, since I think they require some sort of bio for agents listed on the site. Things to look for include: relevant publishing experience (working in trade publishing in some capacity, be it in publicity at a respected publisher or as an intern with established agents at their current agency or, in my agent’s case, as a scout); evidence of industry know-how (sales to big publishers are your easiest giveaway here); and clients (it helps in a big way if you’ve heard of/read even one of them for pleasure). You might only find client lists with names you recognize for established agents, but that doesn’t mean you should discount newer agents! Jennifer Laughran, a most excellent agent with the Andrea Brown Literary Agency, breaks down things any agent needs in this blog post. (Her entire Tumblr and former blog are wealths of knowledge.)
4. Look for sales. If agents are very, very helpful, they’ll list some of their sales on their website or blog—sometimes even in their bio. That’s not the only way to find them, though! You can search on Google, clients’ websites, or Twitter—usually in the form of Publishers Marketplace deal announcements under the Photos tab—for information. Ideally, agents I queried would have plenty of sales to big publishers, and this would be super easy to find out. Sometimes that wasn’t the case and they still ended up on my query list, though. Why?
5. If lacking sales, look for: employment at an agency with good sales, experience at a well-known publisher, or a wealth of industry connections. This last one can be hard to discern, but if the agent has either of the first two, they probably have access to editors.

Other things:

1. Subscribe to Publisher’s Weekly. Now. Not kidding. I get the PW Daily email every weekday morning and the PW Children’s Bookshelf email twice a week, and I read all of them. There’s usually a section in PW Daily with job moves, and this can be invaluable for finding out who joined which agency from where, who got promoted, and who’s leaving the business. PW Bookshelf lists new book deals with each issue, typically in the format of “[Editor] at [Big Fancy Publishing House] has bought Title, an [age category] [genre] [novel/nonfiction work] by [Author]. In the book, [quick plot summary]. Publication is slated for [season] [year]; [Agent] at [Agency] negotiated the deal for [world/world English/North American] rights.” (Sometimes the information is moved around, but usually it’s all there.) This is, 99.9% of the time, a source for legitimate deals to respected publishers with advances paid to the author. Sometimes a less-than-amazing small press will sneak its way on to the end of the list, but that’s pretty much negligible.
2. Read up on terms you’re unfamiliar with. Don’t know what world or world English or North American rights are? Kristin Nelson of Nelson Literary Agency has a great explanation here—I recommend reading as much of her Pub Rants blog as you can. Need some examples of big or respected publishers? Here are some (not all) that publish YA, though the article is outdated (Penguin and Random House are now Penguin Random House, unfortunately not Random Penguin; and HarperCollins has purchased Harlequin and its imprints, for two examples that jump out at me). Wait, what are subsidiary (or just “sub”) rights? Surprisingly, Wikipedia has a relatively good breakdown, though obviously no analysis is given. Google is your friend here!
3. Follow people who are involved with/work in the industry on Twitter. Agents! Editors! Authors! Interns! Publicists! Bloggers! Marketers! Sales people! Those brave souls in the contracts department! Me! We’re fun, hilarious, book-loving nerds. You’ll learn a lot by osmosis. And also, y’know, by the discussions we have on book-related topics.
4. Seriously, follow me on Twitter.
5. Do it.
6. I’m waiting.
7. *stares*