"I have resolved not to think of rainwater as poison falling from the sky, but to re-conceptualize it as freely available water that requires no more purification than does the water from a mountain brook."

Here's one simple thing that you could do.
and to be honest - I will probably do exactly the same thing.

Collect a gallon of rainwater - by whatever method you choose.
Clean a large saucepan - so that it is spic and span.
Pour the rainwater in the saucepan, and heat slowly on your stove.
Allow the water to boil away completely, and take a look at what is deposited on the surface of the saucepan.
That might be a simple place to start - to see how clean the rain is.

Let us know if you find anything. I am curious.

Pete