How do you know you're getting 20 gallons?

I filled up the gas tank this morning. I wondered to myself whether I REALLY had the 20 gallons that the pump read-out said I had. The gas gauge on my car doesn't tell me how much petrol is in there. (I wonder if any carmakers now include an option for a dashboard digital read-out of what's in the tank?)

All gas pumps bear stickers which declare they've been checked for accurate delivery. But exactly HOW accurate is that?

Look what happened to a motorist in China:

A Sinopec gas station in Anhui city of Suzhou province pumped 45 liters of gas into a 35 liter gas tank

Real person real story, yellow plate-less [Chery] QQ, 0.8 L displacement, dealer and manufacturer confirm gas tank is indeed 35 liters.

May 4th, filling up gas. The car still had a little fuel, but the pump kept going until it reached 335 kuai[RMB]. Surprised, thought gas prices have increased to 10 kuai [per liter], because in the past it was always a 200-some kuai.

This car owner is from Suzhou, and hasn’t driven the car out of the gas station after filling up, and instead has been seeking an explanation from the gas station, but there hasn’t been a reasonable explanation up to today. On the afternoon of May 5th, Sinopec brought the people from Bureau of Quality Inspection, as well as from television news. The Bureau of Quality Inspection checked that gas pump and then surprising said there was nothing wrong with it! So a gas tank can suddenly become 10 liters larger out of thin air?

Asked the gas station to drain the gas on the spot [to measure it], but no one from Sinopec accepted [the challenge], all of them hiding.

Still have the receipts, and some replies to what some friends have asked.

No [separate] gas canister.
I pumped gas directly into the [car's] gas tank.
Everyone can check for themselves the gas tank capacity of the 0.8L QQ.
How did it add up to 335 yuan?