Ha..... I actually HAVE sent water in from my local walmart to Ward Labs - three times over the course of a few years! I just don't trust "the man." All 3 times though, it came back close to zero's just as RO water should. So, I consider it good at this point.
Also, you can get a a TDS meter - they are relatively cheap and accurate and will tell you if it is truly RO water or not. Ultimately, all RO water is the same if it is truly being RO filtered.
I primarily get RO from Whole Foods, mostly because they have a two spout machine and I'm typically getting ten gallons, I also use another local grocery store. I always check any RO water I get with a TDS meter and it has always been in single digits. In the beginning I did send a sample of the Whole Foods water off to Ward Labs and it tested as it should.
Yep, the TDS meter can provide some peace of mind for not much money...