From the New York Times:

Where to Find Tests and Testers

To find a certified technician to determine whether radiation or radon is emanating from a granite countertop, homeowners can contact the American Association of Radon Scientists and Technologists (aarst.org). Testing costs between $100 to $300.

Information on certified technicians and do-it-yourself radon testing kits is available from the Environmental Protection Agency’s Web site at epa.gov/radon, as well as from state or regional indoor air environment offices, which can be found at epa.gov/iaq/whereyoulive.html. Kits test for radon, not radiation, and cost $20 to $30. They are sold at hardware stores and online.