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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.)

Service stations intending to defraud might be more likely to adjust their volume measurements than to mix their octanes? Either way, I don't see why one should trust the local station more than this service.

They could give you the receipt or scan it and send it to your phone..