As in any similar problem: you can sample a reasonably representative set of accounts, review them thoroughly by manual means, and see how these manual results

As in any similar problem: you can sample a reasonably representative set of accounts, review them thoroughly by manual means, and see how these manual results compare to the automatic ones. They don't need to know what the total is, if they knew the problem would be solved. 96% represents the probability of a correct prediction, not a fraction of the total.