You can conduct such just an experiment. It is better to refuel the car yourself, do not trust others. After all, you will not be difficult to get out of the car and hold the gun for several minutes?! Follow the moment when the gas station is turned off, only after that you should turn off the gun and remove it from the tank door. At some gas stations, counters are faulty. This is quite difficult to trace, but possible. Often the meter's readings diverge from the actual liter in the tank. But in the tank itself it is difficult to keep track of, since your eye will not be visible to your eye. You can get a canister for five or ten liters, season it. She must be flooded to capacity. Then it will be seen whether gasoline is added to your tank or not.