How would you verify that a database backup is usable?

Restore it into an isolated environment with compatible software and required encryption keys, then check structural integrity and important application-level invariants. Verify expected tables, representative records, timestamps, and recovery procedures. Measure restoration time against the recovery-time objective and confirm the recoverable point meets the recovery-point objective. Include logs needed for point-in-time recovery when applicable. A completed backup job and a file checksum only show that a file was produced consistently; they do not prove that the organization can restore a functioning application.