You must test failover before you need it. Untested failover is not reliable failover.
Test methods:
Simulate failure: Shut down interfaces, power off devices, or pull cables. Watch the network converge.
Measure convergence: Use continuous ping to count dropped packets. Calculate actual downtime.
Verify traffic flow: After failover, confirm all services are reachable.
Test failback: Restore the primary and verify traffic returns (or stays, depending on preemption settings).
Document results. Repeat tests after any changes.