Hell, by the way, is accused of adding up to 5% water to their gasoline. For math wizzes out there, that means for every 10 gallons you're getting a half gallon of water. That's why you have to use so many additives down there to keep your car running as it should.
If you want to know if your getting watered down gas, fill a clear container up and wait for it to separate. If they're adding water, you'll see the water settle at the bottom.