I would find a Baja style filter to catch the debris and water, rather than test it.
The Baja filters tend to be fairly slow to fill a large tank
I have seen setups where folks use hose and clamps to install a Raycor 1000 or two on the fill nozzle to allow a faster fill, and catch much of the bad stuff.
If your tank has a bottom drain , after an overnite much of the water can be drained.
If its a real fuel tank, not just a fuel box, the sump will be a lifesaver.