We had our houe tested by the home inspector for about $80. He placed 2 canisters in the house, one in the basement and one in the living area, for about a week. The canisters were then sent to a lab for analysis, it turned out very low in radon gas. Just for the peace of mind in the long run, we bought a plug-in radon gas detector (just google Safety Siren pro) for $100 and we monitor it everyday.
Remember, just like someone else mentioned, just because your house shows low concentration on radon gas does not necessarily mean the same thing for your neighbor next door.