Use databases created by credible sites such as the Association for Author Representatives (AAR). The AAR database has a searchable list of literary agents with search parameters that can be tailored to your writing genre. Each listing has complete contact information if the agent is accepting new clients and submission guidelines (see Resources below).
Pick up a copy of a credible publication such as "Writer's Digest" or "The Writer's Market," which outline thousands of literary agents by genre. Each listing includes submission guidelines, contact info, background information on the agent and inside information in the form of tips that aid an author's submission. "Poets and Writers" magazine also profiles literary agents in search of new clients.
Browse the dedication and acknowledgment pages of books similar in style and theme as your manuscript. Authors often list the name of their literary agent or agency in these sections. When writing your submission query, try to highlight some of these similarities to give the literary agent an idea of what to expect from your book.
Check that name of the agents that you choose against websites such as the Science Fiction and Fantasy Writer's Association's "Writer Beware" Web page. Through real feedback from authors, the site compiles a list of agents involved in fraudulent behavior (see Resources below).
Look for entertainment attorneys who double as literary agents. These agents combine the services of attorney and agent to offer better services to authors. You can find these attorneys amongst general attorney listings, including those in the yellow pages. Be sure to interview this agent before signing a contract, because not all entertainment attorneys make good agents.