For the public's protection, Congress enacted the Clean Water Drinking Act in 1974 and strengthened it in 1986, setting minimum water quality standards for most homeowners. Unfortunately, however, there is no guarantee that all utilities comply with the regulations. It's also somewhat reassuring to know that, when something does go wrong, utilities are required to notify their customers.
Wells are another story.
Private wells and systems that serve fewer than 25 people or 15 service connections are regulated only by state and local laws or by their owners. The only way to be sure that your water has certain harmful pollutants is to have it tested by an independent lab. Unfortunately, you must test for each type of pollutant separately, which can be very expensive. Nevertheless, it may be a good idea to check for lead because lead can leach into the water from old lead-based plumbing. You can also discover plenty of information without testing.