You can do it 2 ways depending on if the customer has power on the relay.
If they have power on the relay, take the black lead of your meter and place it on the ground lug of the cabinet and place the red lead from the meter on the line side to see what the voltage is. Then switch the red lead from your meter to the load side and toggle the relay using the HOA switch. If the relay is working, you should see the voltage change from 0 to whatever voltage is supplied.
If they have not landed voltage to the relay, you can check continuity across the relay using the ohm setting on your meter.