At this time, one 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 multiple 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.
Although not specifically designed or intended for measuring radon emissions from countertops, do-it-yourself home radon test kits are available through retail outlets and online. If you are concerned about the radon level in your home, purchase a test kit and use as instructed. The Surgeon General and EPA recommend testing for radon and reducing radon in homes that have high levels. For more information on radon testing, see Find a Radon Test Kit or Measurement and Mitigation Professional on EPA.gov.