Connect your meter leads to the two flat prongs on the cord, should read less than 1000 ohms if everything is ok. If it reads infinite remove the knurled nut from the block heater and pull the cord out, connect your leads to the two furthest apart prongs on the heater, should read less 1000 again, if so then the cord is bad, if infinite the heater is bad.
99% of the time the cord is the problem but is hard to detect with a meter because just moving it around can close any breaks in the wire. Cord is available separately from Cummins or the dealer.