At this time, a generally accepted radiation testing protocol for countertops does not exist, and neither imported nor domestic granite products require radiation testing.
Radiation concentrations can only be measured using sophisticated portable instruments, or with laboratory equipment. These instruments and equipment require a knowledgeable and trained user and proper instrument calibration. For information about local radiation experts, the Conference of Radiation Control Program Directors (CRCPD) maintains a web page where you can find contact information for each state's radiation protection program - www.crcpd.org/Map/map.html
Although not specifically designed or intended for measuring radon emissions from countertops, do-it-yourself test kits are available through retail outlets and online, starting at about $25. If you are concerned about the radon level in your home, purchase a test kit and use as instructed. Learn more about testing your home for radon or read, "A Citizen's Guide to Radon: The Guide to Protecting Yourself and Your Family from Radon."