INDESIT BAN 12 test procedure

Does anyone know how to test a ban 12 to check to see if the defrost heater is working.Fitted pcb 5 weeks ago wevgot called out to this nightmare it was choked with ice as we had already advised the customer to defrost the previous week i started at the heater it was fine so i thought must be pcb replacd the pcb choked up three weeks later its starting to do my head in.