This page shows FDIC insurance on trust accounts is subject to the number of beneficiaries. Yet a bank will open a trust account given only the number of trustees, not the number of beneficiaries. An account with two trustees and one beneficiary might have $5m in it, a good amount of which the FDIC won't cover. So how does the FDIC actually know the correct grand total amount in all banks it is currently liable for? (I'm guessing they don't really know.)
The FDIC periodically checks to make sure the bank is accurately accounting for all the deposits in the bank through a compliance Reporting Review
verifies whether the insured institution accurately calculates and reports data upon which the FDIC assesses deposit insurance..... issues a report discussing the institution’s compliance with assessment reporting requirements, the effectiveness of the institution’s documentation, and the institution’s responsiveness throughout the review process.