Philadelphia tap water and reefs...

If I remember correctly, municipal water supplies are suppose to have a water quality report available to the public. It may be posted on the internet (check the city government website), or you may have to request a paper copy. That would be a good place to start. Some municipalities test for more parameters than others. As a result, the report may not contain concentration levels for all of the parameters you may want to consider before using the tap water in your reef tank.

Otherwise, a simple, broad spectrum measure of your tap water quality is to check it with a TDS meter (Total Dissolved Solids). One can be found online at various places for around $20 if you cannot find someone with one you can borrow. It will not tell you what is in the water, but it'll provide a lump sum total of what's in there. The closer to 0 the reading is, the better.