in a simple way, disconnect the breather hose from the inlet fitting and put the hose into a gallon bottle half full of fuel. Set the hose so it is just off the bottom. Any expansion or contraction of the fuel in the tank will cause the fuel in the bottle to rise and fall. The air with the moisture will be on the outside of the bottle and any moisture will be trapped in the bottle fuel and not passed into the tank. The system is called a "conservator system" and used to be used on all large AC power transformers prior to the industry using nitrogen blankets . The systems were a bit more complicated then a gallon bottle of fuel but the idea is the same. We would go by the transformers once in a while and drain out the water in the conservator tanks. Then we would test the transformer oil (very similar to diesel) and it would have to pass 35Kv over 1/8 of an inch gap. That's 35 thousand volts over 1/8 inch gap so you know the oil was dry. It's very, very simple.