Find out how often the water is tested, what it is tested for and whether any violations are on file. Also ask about any known hazards--such as lead-- that might enter water between the treatment plant and your tap.

If local hazards exist or if your home is served by a well or small utility, have your water tested by an independent lab.

You can ask your utility or the state health department for names and numbers of labs, or look under "Laboratories" in the Yellow Pages.

Another option is to contact a national laboratory that specializes in water analysis. National Testing Labs (800-458-3330) or Suburban Water Testing (800-433-6595) will send you instructions and a water sampling kit that you mail back.

A lead-only test costs about $35; a comprehensive test for organics, inorganics and bacteria runs from $137 to $167, depending upon the range of the tests. Some filter makers offer this service, too. For example, you can call the GE Answer Center (800-626-2000) and request a water test kit and information on GE's SmartWater line of filters and treatment devices.

Be aware that a large municipal system's water quality can change daily, because it may draw water from more than one source. Also, chlorine levels --which affect the amounts of bacteria and some other toxins-- can fluctuate hourly.

If your water comes from a well, you should do several tests. Test your well once for lead and radon, if radon is suspected in your area.

Once a year, test for bacteria. And, if specific pollutants such as pesticides or gasoline may be a problem in your locale, also test annually for these (neighbors can often share the cost of this testing).

Once you have reports or test results, compare them with the EPA's maximum allowable contaminant levels. For a free copy of these listings, call the EPA's National Safe Drinking Water Hotline at (800) 426-4791.

When you compare, note any pollutants that exceed federal standards. Record these, then use this list to select an appropriate water treatment device.