How do we know that water used for making soda drinks doesnt contain bromide?

Answer 1

contact the place where the product was bottled, ask them where their water supply comes from, then contact the water board or authority in that local for their test info on the water purity, it shouldn`t differ one iota to whats on the product lable, if it does, contact the consumer health people in your area, they can advise your next step.

Answer 2

call the manufacturer.of course they might not know or will lie.
two people in my life. I have heard .that they died from drinking to much Pepsi ,and the other coke,anything can be over done, I reckon