Glacier Water is the primary vendor of RO water machines outside or inside of supermarkets in Northern California; and I had the chance to test a few of the machines over the course of ten, long, dry days while I replaced my RO set-up.
Machines all within minutes of one another varied a good deal in terms of "purity" and ranged in number from 5.5, 6.9, 12.4, and 18.4 ppm, to well over 60 ppm at others.
An inexpensive TDS meter seems like a good idea . . .