There's no absolute way of knowing. Of course, it's the same with physical books. There's no absolute way of knowing, short of selling them yourself.
If you can't trust your vendor/distributor/store, you should likely look for one that you can trust.
Audits only go so far and do cost money and if you sick an auditor on a company, how likely are they to do further business with you in the future? Just another data point to consider,