Andrew, as Deb suggested already the best way is to check your TDS.
I bought a handheld TDS years ago when I got my RO/DI and it's paid itself off a housand times. For $30 a meter can check your main water line, RO, DI, and waste water lines, heck if your bored you could even buy a few brands of bottled water to see who's got the cleanest H2o not that I have done this of course
