Actually, hot spring water usually *does* show a subtle, if not significant, difference in color as compared to normal hot water.

Referring back to my [previous answer][1] of how in Arcanretia, hot spring water vs normal hot water is analogous to *onsen* vs *sentō*, if you were to look at stock photos of *onsen* baths, you could see that the water is somewhat cloudy and unclear:

[<img src="https://i.sstatic.net/Otu9Q.jpg" width="320" height="320">][2]  
<sub>(source: [hikyou.jp](https://www.hikyou.jp/img2/R1555723592943a_FWx800_5557255519bf4.jpg))</sub>  


You can probably tell the difference between that kind of water and clear, normal hot water.


  [1]: https://anime.stackexchange.com/questions/47214/what-is-the-difference-between-hot-spring-water-and-regular-hot-water/47220#47220
 [2]: https://i.sstatic.net/Otu9Q.jpg