You could send a sample to one of the labs found online. There's one the guys here use a lot I forget the name of though. I'd say at least filter it & test the PH.
Definitely get it tested (Ward Labs?). In addition to concerns with acidity from power plant emissions, any rainwater gathered from roof gutters can have additional contaminants, especially the first runnings:
"Due to the acidic nature of ambient
rainwater, chemical compounds from roofing materials may leach into the harvested rainwater
(King and Bedient, 1982). Specifically, heavy metals such as cadmium, copper, lead, zinc, and
chromium have been detected in rooftop-harvested rainwater (Quek and Förster, 1993; Lye,
1992; Yaziz et al., 1989)"
"Generally, the first flush contained the highest concentrations of microbial and chemical
contaminants in comparison to the subsequent collection tanks, indicating that the quality of
harvested rainwater improved with roof flushing. However, the rainwater harvested after the first
flush did contain some contaminants at concentrations above United States Environmental
Protection Agency (USEPA) drinking water standards (i.e., turbidity, TC, FC, iron, and
aluminum). This indicates that harvested rainwater must be treated prior to potable use."
http://greywateraction.org/wp-conte...-Quality-for-Rainwater-Harvesting-Systems.pdf