A validity check is a query that must return zero rows:
SQL
-- Orders must reference a real customer.
SELECT o.order_id
FROM orders o
LEFT JOIN customers c USING (customer_id)
WHERE c.customer_id IS NULL;
Anything it returns is the incident report — the failing rows themselves, not a boolean.
Important
Alert on the check's trend, not only its first failure: ten bad rows a day for a week is a different bug than ten thousand at once.