The Audit. Marking your own homework does not count.

An audit is a stranger with authority checking work that somebody had every incentive to flatter. Companies are audited by licensed firms. The Government of India is audited by the Comptroller and Auditor General, who reports to Parliament rather than to the ministries being examined. The independence is the entire product.

• Audits fail when the auditor is paid by the audited and hopes for the work again next year. Rotation and separate reporting lines exist to break that link.
• The same logic works personally. Before anything important, ask somebody who gains nothing from the answer to check it.