Our area has a karst topography, which often contains small amounts of uranium, the parent element to radon gas. There is no way of "guessing" whether or not a home will contain radon gas, a known carcinogen affecting the lungs. The only way to know the concentration of radon gas in a home is to test for it. Numerous methods can be used to test for radon gas, but we use a continuous radon monitor which "counts" the number of alpha particles that have been released into the air via radioactive decay. A reading of picocuries of radon radioactivity per liter of air is the result, and we analyze the data and give you direction on whether or not mitigation is recommended.
Newer homes will often employ a passive system, meaning that a pipe is in place that extends from below the basement floor to the outside of the building envelope. It has been our experience that few of these systems are effective at bringing the radon down to a safe level. The good news is that mitigation costs will be minimal with this piping already in place, only involving installing a fan... But we don't know without testing!