Well, if you live in Chicago, you probably never know where your city water is coming from, day to day, I suspect.
I live in Helena, Montana, and know my city water is coming from the Tenmile reservoir system 98% of the time, so I don't have to worry about it.
Now, on occasion, when the Tenmile water treatment plant is out of commission, my tap water comes from the Missouri River. I instantly know it from the mossy smell (I don't need a pH meter to tell me i have different water). I refuse to brew with that water, I can drive up to the Continental Divide where the highway department has developed a spring where I can fill plastic carboys. And that water has virtually the same cation/anion balance as my Tenmile city water.
I admire you guys who brew in less pristine parts of the country. I would brew with RO water if i lived in the Deep South - their tap water tastes terrible.