In 2008, 195.5 million people (72.4%) on public water systems had fluoridated community water, which is a substantial increase from the 2006 figure of 184 million people (69.2%). The easiest way to know if your water contains fluoride is to check with your local water supplier, or use this interactive click map from the CDC.
The 1991 study on fluoride conducted by the US Department of Health and Human Services proved inconclusive,
Fluoride: The Hidden Poison in the National Organic Standards
Fluoride Action Network