Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
CH Demand
Remove screw from middle of pump (use tray to catch any water)
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Allow pump to run. Check pump is spinning. Then turn power off. Watch the pump, does it stop spinning within 2 seconds.
Or does it continue to spin longer than 2 seconds indicating that the impellor is loose.