Unfortunately, most writers can't make it to conferences. So, they are left with the task of writing query letters. (Even if you do find an agent at a conference, you are not off the hook. You still have to write a query.)
Whether you meet an agent in person, or look one up online, you have to do your research. Does the agent represent your genre? Is he/she they willing to take on new clients? Does she/he have a good track record for sales? Which publishers has the agent worked with? How does he/she treat clients?
All of these questions are important, and all can be answered by researching agents online. These are the steps to take for researching an agent.
1) Go to AgentQuery. Not all agents on agentquery are members of the AAR, but all are established. You can search on an agent's name, or you can search by genre. If you search by genre, make sure you do a full search and filter for agents who are actively seeking clients. As you find agents who are taking on clients in your genre, make a list. This site has lots of great resources, including a list of twitter handles for agents, how to write a query letter, and lists of publishers and literary magazines, to name a few.
2) Go to the agent's or agency's website. (You can do this directly from agentquery.) Is the agent still taking clients? (Sometimes, agentquery is not up to date.) Does the agent still represent your genre? How does the agent sound? Does he or she appear helpful. and supportive? Does the agent charge a "reading fee"? (If so, cross the agent off your list. No reputable agent will charge a fee for reading a manuscript. No reputable agent will use a submission as an excuse to offer "editing" services either.) Very few agents post their contract on their websites, but when they do, read those contracts carefully. Literary agents, unlike talent agents, do not represent you, they represent your book. (Only one.) Contracts that demand representation rights for all your work, or for prequels or sequels, should be avoided.
3) Google the agent. Look for "querytracker," and "absolute write" entries. Both of these will give you insights as to turnaround time, and whether authors have had positive (or negative) experiences. You can also go straight to querytracker and look the agent up directly, and you can go to absolute write forums. (I find it easier to do a google search.) If there are interviews posted online, read them. Interviews are a great way to discover what the agent does and does not like in a query letter, which genres he or she is particularly interested in, and other useful details.
4) Look the agent up on Preditors and Editors. The Pred and Ed website has an extensive list of agents and publishers. If misconduct has been reported, they flag the agent as "not recommended." They also flag the agent if he or she is recommended, and if there have been recent sales. This is a great website, with a lot of resources, so while you are there, feel free to look around.
5) Google "agent [agent's name]". When authors publish a book, they almost always thank their agent in the "acknowledgements" section. Often you will find something like this: "I want to thank my agent, [name of agent] for her tireless support and encouragement." If the book has been listed on google books, this search will find all such acknowledgements. This is a good way to see how many books the agent has represented, which is particularly useful if the agent does not include a list on his or her website.
Once you have assembled a list of agents that meet all your requirements, it's time to send queries.