Starting in s02e09, Aqua is accused of purifying the hot springs in Arcanretia. In the meta sense, we can tell the difference by the animated color (green for hot spring, clear/blue for regular water), and that Aqua is known to be able to purify water simply by wading into it, as we learned in s01e05.
It's never mentioned how any of the citizens would know that. Here in reality, mineral water is the same color as tap water.
How would the citizens of Arcanretia know that the hot spring water was purified into regular water?
Note: This is a followup question to What is the difference between hot spring water and regular hot water?, not a duplicate of it.