#1 It's RO water. I specifically buy RO water. I want it filtered to death. I'm not looking for "spring water". I know that I am buying city water that has been run thru an RO filtration system.How do you know the water you purchase is better than the tap water?
#2 In NM, the dispensers are tested by the local health depts to make sure they are filtering and changing filters like they claim. We used to run a food cart and we knew one of the heath inspectors quite well plus we were buying our ice from a guy who was fanatical about the local water, we learned a lot about water from him). I'm assuming all states do that. We vended food in NC, GA, TX & NM. Health Depts required us to use ice and water from tested/certified sources. TX has the 2nd toughest food service rules in the US (CA being #1 but many of their rules are idiotic and are aimed at lining someone's pockets). When we built our food cart, we built to TX state code for food trucks and then operated per the same guidelines even though we were in NC at the time.
#3 Unless they are making mega lies about their filtering, Twice The Ice, Windmill Express and Primo/Glacier all have the info on their websites. I always check out the company that brands the water dispenser. The internet is great for that. The three listed are pretty much the only ones I use. We knew a Twice The Ice machine owner in GA where he told us all about the filtration. They had sucky water there. It either smelled like the nearby river (fishy) or it smelled like pesticides. And I kept getting sick from the water. That was when I built my first filter system which consisted of a spin down sediment filter and a DB2 filter that I chose because it filtered out the specific pesticides they were using on the crops we were surrounded by. It wasn't until the giardia episode that we completely switched to RO water by the jug. My little filter combo works okay for water we don't ingest.
Twice The Ice Twice Pure Water - Refreshingly Pure, Purely Chilled - Twice the Ice
Windmill Express Water – Watermill Express
Primo/Glacier Primo® Water Refill Stations | At Grocery Stores Near You!
It wouldn't be hard to get better water than local ground water. There's a lot of nasty stuff in NM water. The water has so much lime in it that it EATS metal, you replace faucets, shower heads and water heaters every couple of years. With all the old abandoned mines, oil wells and dairy/feedlots, there is no telling what's in the water.
And it's not just NM. We knew ahead of time to not drink the tap water in Lubbock TX. We went over for a few days with a full fresh tank ("home" park water run thru a filter) and filled the 3 gallon jug we keep in the truck camper before leaving. I also learned to not drink anything that could possibly be made with the local unfiltered stinky water (the water smells like an oil field and the colour is... off). The ice tea was horrible. I carried a refillable bottle of water and my little raspberry tea water additive everywhere, including into restaurants. They weren't surprised.
As for "city" water, I know what my 0.5 micron filter looked like when I would change it. The town below us has "boil" warnings several times per year. For Roswell, it tends to be "oops, too late, that would have been for yesterday, never mind, carry on". And the water lines break pretty often, especially in the winter because they are so shallow.
The TN park where my dog and cat got sick off the so-called potable water, never published that they had bad water. We were still in the area for several months after we stayed there. I used to live near a park in NC that would fail the water test several times a year, every year. We knew because if a park failed their test (they always had giardia cysts) the park had to post a notice in the local paper. Like my husband once commented, it wasn't much help to the folks who filled their tanks and left over a month ago. Occasionally we would run into someone who stayed there and we would comment on the failed tests. They were always horrified and ignorant of the failed tests. And they were often having a bit of "stomach flu". I can understand their shock. We lived in an area that bottled and shipped "pure mountain spring water" regionally. Most wells and springs had great water.
Like I said before, you do what you are comfortable with. Water purity is a personal thing. This is what works for me for now and I have given all the reasons and explanations for my choices. I may have just been unlucky with my water. Our home well in FL had a slight amount of iron and lime in it and I used to drink water from the artisan wells in the orange grove, which tended to be mildly sulphured. That kind of water is okay for me. Well, maybe not any more since I haven't drank tap water from anywhere, except my Moms house in NC, in decades. It's the pesticides, poisons and cysts that I object to. And I'm not going to search for a water quality report for someplace that I am passing thru.