Radon is a naturally occurring and radioactive gas that is present in nearly all soils. Levels can be high in any state of the U.S. and only an indoor air test can confirm the levels. Your state health page may have free testing kits available for use.
On remodel projects, testing can be completed in the existing home. With new construction, checking with local resources through the EPA for radon maps can be used as a starting point. Speak with your builder to ask if they have experienced radon in their prior builds in the area, or if they integrate radon mitigation in your home. Your architect can update your plans to reflect the integration process.