There are typically no sources of water that are free and clear of contaminants. However, a water quality test is a simple way to check that the water is not hiding any hazardous pollutants.
A water quality test can reveal if there are dangerous bacterias in a water supply or higher than normal mineral deposits like fluoride. This test can also look for heavy metals like lead or iron and even give you an idea of how hard the water is in the home.
Water quality tests are simple and can be done on either public water supplies or well water systems. The EPA can give you some other factors to consider when trying to determine if a water quality test should be done in your home.