A couple of years ago I filled my scooter up with petrol from a tank that had water in it. It caused no end of problems. Anyway I have never been back to that station since and the one I go to now checks the tanks for water at opening time.
I guess they are using a tool as mentioned in this article...
https://www.nbcnews.com/business/consumer/watered-down-gas-way-more-common-you-may-think-n776016I quote "The station may also physically test the gas for water with a tool that turns from white to burgundy when it detects water."
It's good to know that they check for this as its a nightmare if you get any water in your petrol.