There are several types of low cost ($15-$20) radon test kits you can get at your local hardware store. These kits will often be the passive variety and will most often require you to send them to a testing laboratory.
There are active systems that can test for radon as well. These systems are radon detectors that plug into a standard AC outlet. They can perform continuous radon gas monitoring.
You can also contact a private radon proficiency program for lists of privately certified radon professionals serving your area. For links and information, visit www.epa.gov/radon/radontest.html.
The US EPA has established a guideline for do-it-yourself radon test kit accuracy of +/- 25%. This equates to +/- 1 picocurie at the EPA’s action level of 4 picocuries per liter of air in the home. This level of accuracy is easily accomplished when using most do-it-yourself devices, IF the user carefully follows the instructions.