A couple of the answers below correctly advise you to interview the agents you're considering. Absolutely. And the points made about the listing agents being more important, overall, in the scheme of things is also correct.
Still, when you interview them: Ask. Ask "What experience do you have in assisting buyers who have purchased short sales? How many transactions have you been involved in while representing buyers? What can you, as a buyer's agent, do to assist me in purchasing a short sale?" And ask the same questions, substitituing "foreclosure" for "short sale," to find out their foreclosure experience.
And a side note: Not all short sales and foreclosures are good values. Don't limit yourself to those two categories. Consider "conventional" sales as well.
Hope that helps.