In the past we have used a drill pump with a long bent copper tube that reaches the bottom of our fuel tank to pump out bottom fuel into a clear glass gallon pickle jar. We would than let the jar sit undisturbed for several days and then look for water at bottom and growths between water & diesel layer. We have done this pump several times over many years and have never seen anything unwanted.
Recently on the forum I learned about Water Finding Paste that can alert to bottom water in fuel tanks. We purchased a 4 oz tube of Gasoila AP02 and smeared it on a flexible piece of long plastic molding. Glad to report that the brown past color did not change color indicating no water found in diesel fuel tank.