RiverLady,
A couple of things. One, there is a set of laws called the Safe Drinking Water Act (SDWA). It describes the level of contaminants that are allowed in drinking water. You're municipality and the bottled water sellers must both meet those standards. The differences between tap water and bottled water are miniscule. Primarily, it has to do with taste and odor due the chlorination of tap water.
Two, go to your county water department's webite. Usually they have their annual water quality report posted there.
Three, do not try to test the tap water on your own unless you are well versed in the sampling protocols. To sample for all of the expected contaminants (metals, biological organisms, fecal matter) which are regulated costs about $300 per sample at a reputable lab.
MikeCJ