According to the latest issue of The Radon Reporter, there are six U.S. states that require radon professionals to be certified (CA, CT, IN, KY, NH, VA) and eleven others that require a state license (FL, IA, IL, KS, ME, MN, NE, NJ, OH, PA, WV).
So how do you find a licensed or certified radon professional? If you live in a state that requires a license, you need to check with your state. Someone there should be maintaining a list. The name of the government agency that handles this task varies from state to state, but here in New Jersey you can check with the Radiation Protection Element of the Department of Environmental Protection.
If you live in a state that requires a certification, you can easily check with one of the two national organizations that certifies people to do radon work. Here are the two national organizations, along with links to pages where you can search for certified people:
National Radon Safety Board (NRSB)
Please note, just because your state doesn’t require a certification does not mean you should hire someone who does not have the credential. Most states have many people who are certified, regardless of the state requirements, and it’s a good idea to work with someone who has taken the time to learn what’s necessary.