Reminds me of Germany in (I guess) the 80s, where police was sometimes checking diesel cars on whether owners had used (lower taxed) heating oil for their cars (which is technically tax evasion). It might still be a thing.
In eastern Europe(Baltics) it is still a thing because people just bring over Diesel from Belarus or Russia by cars with big fuel tanks and low fuel consumption, so they simply cross the border and at the first fuel stop brim the tanks and go back to europe, In Belarus it is especially easy as the fuel price is mandated by government and is equal across the whole country.
And police sometimes checks the fuel tank for non taxed fuel, it is either Green(Agricultural, illegal for anything else), Normal taxed, and Russian(If you can prove that it was "imported" by you then its ok) non taxed.
We pick up avgas (half the price) from the small airport near us if we’re doing strictly off-road activities in our vehicles (no fuel tax).