The only way to know if a house has elevated levels of radon is to test your house. 
There can be three houses in a row, the two on the outside are high (above 4) the and one in the middle can be low (below 4). 
However, much research and data collection has be done and there is statistical data indicating the probability of elevated levels of radon by ZIP code. The information presented below was obtained from the State of Utah, Department of Environmental Quality, Division of Radiation web site. The data on the left side are listed the most populous counties (Salt Lake, Utah, Weber, Washington, Davis & Summit). On the right are listed the less populated counties.