How would I measure the pH? I'd just go to the local store and get some litmus paper, to be honest. They have it in the gardening section of the local megastore (Fred Meyers) to measure soil pH. Or, I'd get it from a hobby store or from an online chem supply house.
The pH that I'd want to maintain would be above 9.0 for antifreeze. As for the pH of water wetter coolants, I don't know for sure what the initial pH of a 1:32 (water:ww) liquid is. I also don't know for sure if there are any buffering chemicals in WW. That being said, my first guess is to keep it at pH 9.0 or above.
Being lazy, I plan on changing the coolant fairly frequently (about every 4 months). That's probably MUCH more than I need, but it really isn't that expensive to do a coolant flush. I do have an "inspection port" to monitor the color and transparency, too. If the coolant becomes cloudy or discolored, it'd be an immediate alarm bell to do a flush. I'll also do a quick inspection of the heater core and waterblock when I do my flushes. That way I can increase or decrease the frequency of my changes if things look good.
Besides, this way the feral cats and possums get a quick waterdish treat every 4 months!
Keeps their population down.
P.S. Just kidding about the waterdish, but it is a tempting thought.