Visit the agent's website. Find links that show a list of that agent's clients. To an agent, clients are the reason they're in business. An agent's clients and list of books they've represented solidify his resume. Thus, many agents proudly host a page on their website with lists of books and authors they've worked with.
Email the agent and ask them to provide you with a list of authors and books they have represented and sold to publishing houses. You should never feel awkward when asking an agent questions. After all, you may be potentially hiring them to represent you and your manuscript.
Check the acknowledgments page of an author published in a genre similar to your own. More often than not, an author will thank her agent in the acknowledgments of her book.