The National Ground Water Association (NGWA) recommends homeowners test their water at least annually for bacteria, nitrates, and any contaminants of local concern. 

Small amounts of harmful contaminants, many of which do not smell, look, or taste like anything at all, can cause serious physical and mental health effects as they build up in your body. The key to protecting yourself and your family is having reliable and convenient access to high quality testing.
Check with your local health or environmental health department for recommendations regarding the type and frequency of testing specific to your location.