I'm curious about how the customer knows they are getting the grade of gasoline that they paid for. The stuff that comes directly out of the pump is periodically measured by regulators. What assurance does the Yoshi customer get that they are not getting 87 after paying for 91?
How do you know the stuff at the gas station is actually 91 and not 87, how do you know the regulators do their jobs properly and aren't bought out by Big Oil?
The answer to all questions is that random private individuals or groups do the tests. If it's a big enough concern among their customers, some of them will do it. Some customers at regular gas stations already do these tests on their own because not all gas stations are the same. (A few people even make their own gas.)