Is the tank pretty much straight-line accessible through the fill cap neck? Simplest...calibrated dip-stick. Make a thin wooden dip stick that will reach the bottom for your tank. Empty your tank. Refill the tank, a fraction of a liter at a time. Insert and measure the height of the fuel on the dipstick after every addition. Mark the levels on the dip stick. Numbers for liters and tick-marks for fractions. Next time you refuel, measure the level before and after the fill. Compare your measurement with the station's claim. Analog, not digital, but simple and cheap.
Ken