This video is awesome at explaining how to use a mutli-meter to test and diagnose a circuit problem from the relay panel.
https://www.youtube.com/watch?v=6eZg3wLYsr0
he uses a specific tool called LOADPro you can buy that from here:
Electronic Specialties 180 LOADpro Dynamic Test Lead
Basically the load pro is a high wattage 50 ohm resister with a switch.
