Hmm... sampling fuel--reminds me of the preflight you do on an airplane before you take off.
The Cessna I used to fly, you had to climb up and visually check the fuel level in each tank (the FARs only require that the aircraft fuel gauges be accurate when reading empty

), open the fuel strainer up in the engine compartment for a few seconds (like our fuel filter), and use this neat little sampling tool to get a fuel sample from each tank. Looked like a plastic test tube, with a 1" pin sticking up above the center of the opening. You pushed that pin into a port in the wing, and it opened up, draining fuel from the bottom of the tank.
I've thought a few times something like this would be handy in an auto, since you could check the color (100LL AvGas is blue), smell it, check for water...
A bit easier to do it standing under an airplane wing, though, than crawling under one's truck at the fuel station...
Still, I think a check port at the lowest point of the fuel tank could come in handy for draining, if nothing else. I'm guessing gas and diesel would mix together, but water will settle in the bottom...
--Ty