You could test the pH (acidity) of the collected water with some pH paper. In reality "acid rain" isn't all that acidic. Soft drinks and orange juice are significantly more acidic and the body handles them just fine. Besides, the amount of acid rain failing has been dramtically cut over the last 25 years. Harvest the rain, treat it as you'd treat any "wild water" and you'll be fine.

Oh, and I second the fish method of keeping mosquitoes at bay. I have three 55-gallon plastic drums which I catch rainwater in. One has a metal mesh screen over it and the other two use $1 goldfish from Walmart. I haven't seen a mosquito larva in years and the goldfish are are more fun to watch than the mesh screen.

-Blast