Soft water, as opposed to hard water, is water with little or no dissolved calcium and magnesium. Hard water requires more soap or detergent for laundry or dishwashing, and can leave calcium carbonate deposits on shower heads, boilers, or pipes. For accurate testing of water hardness, contact your water utility. They may provide free testing or already have data on the hardness of your water. You can also check the USGS's map of water hardness in the resources section. A simple test can be done at home using dish detergent. Hard water can be softened with an appliance called a water softener.
How to test water hardness
Fill your container 1/4 full with water from your tap.
Add five drops of dish detergent and put the cap on the bottle.
Shake the bottle a few times. If you have soft water, the soap should foam up very quickly and may fill the whole bottle. If you have hard water, it will foam much less, perhaps only forming a thin soapy film on the water's surface.
Repeat steps 1-3 with distilled water if you are having trouble deciding if there are enough suds or not. If distilled water is not available, use filtered water. If there are more suds in this sample than with your tap water, you have hard water. Otherwise, your water is soft.
Check your faucets, shower heads, bath tubs, toilet tanks, boiler or radiators for signs of calcification. If there is a hard, white scale buildup, you probably have hard water.
Short of getting a water softener, all you can do to repair the damage done by hard water is to regularly remove scale buildup before it clogs shower heads or faucets. White vinegar is a safe and effective remover of calcium carbonate.