Contact the municipal water supplier in your city if your water is supplied by the city. Their contact information may be found on your water bill or through the city's official website under "public utilities/works." They will be able to tell you exactly how hard or soft your water is.
Contact your local county extension office if your water comes from a supply on your land. For a nominal fee, it will test your water and provide a complete report on its hardness and quality.
Purchase a water hardness testing kit (online or at your local hardware store) and test your water by following the manufacturer's instructions.
Interpret the results of any water hardness test based on the classification methods of the U.S. Department of the Interior and the Water Quality Association. Your report will be given in milligrams per liter (mg/l), parts per million (ppm) or grains per gallon. Soft water falls between 0 and 17.1 mg/l and ppm or between 0 and 1 grains per gallon. Slightly hard water falls between 17.1 and 60 mg/l and ppm or between 1 and 3.5 grains per gallon. Moderately hard water falls between 60 and 120 mg/l and ppm or between 3.5 and 7.0 grains per gallon. Hard water falls between 120 and 180 mg/l and ppm or between 7.0 and 10.5 grains per gallon. Very hard water is classified as anything over 180 mg/l and ppm or 10.5 grains per gallon.