As the others already said: You had a dipstick. You knew how much you've driven since the last time you refueled. And even if you just forgot everything else

As the others already said: You had a dipstick. You knew how much you've driven since the last time you refueled. And even if you just forgot everything else there is a lever on the passenger side called "Reserve". Pull that and you have about 5 liters more. That should get you to the next gas station.