In the UK if you get fuel from an airport fuel supplier, it is law that the supplier MUST test the fuel in the system everyday before the first delivery and after the last delivery. They use Shell water detector capsules. A syringe sucks the fuel up through the detector and if it turns green the fuel contains above 30 ppm of water or more. Not good.
I suggest if you are worried about water contamination then you get hold of the US equivalent and use them before filling your tanks.
I use them every time I fill my model just to be safe.

I would go to your nearest airport fuel supplier and ask to buy a tube of them and a syringe.

Check out

http://www.heli-basesupport.com/fuel_testing.htm

You will see them on the right of the screen