According to this pdf[1] they may take soil or tissue samples, but primarily the inspection leans on self-reporting. I’d wager that the (3rd party) inspector is mostly checking that you are following your Organic System Plan. There is also nothing stopping the certifier from sending out a separate consultant the week before to make sure their customer has all their ducks in a row. I know thats a thing for ISO certificates.

Fwiw, most regulatory agencies I’m familiar with in the US rely heavily on self-reporting.

[1] https://www.ams.usda.gov/sites/default/files/media/Guide%20t...