The ai-bookkeeper sales tax return was tested on hand-written invoices. Today it also gets a seeded property test (salestaxreturn-property.test.mjs): 150 made-up ledgers of invoices and credit notes with up to three tax jurisdictions each, run at monthly, quarterly and annual frequency.
What it checks: the tax over all periods equals the signed tax lines typed in, whatever the frequency; payable is tax minus discount; period totals equal the sum of their rows. A second test runs every state's due-day rule for monthly, quarterly and annual periods and checks the due date falls after the period and, once moved, on a weekday that is not a federal holiday.
It passed first time, and a deliberately broken credit-note sign made it fail, so it does catch something. It does not check any state's actual due date: state tax sites refuse this session's network, so those dates still rest on the sources named in salestaxreturn.mjs. Check your own state notice.