This is something easily testable. You can buy a cheap yet accurate pH tester online. The next time it rains, pull a sample and test the pH. If it’s less than 7, then it’s acidic. The lower the pH, the stronger the acid. The thing is, even purified water is listed in the USP (United States Pharmecopia) has having an allowable pH range of 5 - 8. Reason being that typically any acidity or basicity present is going to be easily neutralized with exposure to air.
I work in the water testing industry, these test strips are so badly off that they’re practically useless for anything other than a ballpark measurement. Litmus paper will really only tell you if it’s less than 7. There are digital pH meters on amazon for about the same price that are much more accurate. https://www.amazon.com/dp/B01M5IASHD/ref=twister_B0CT8QKCVB?_encoding=UTF8&psc=1 Is one i’ve used for a similar purpose as the poster.