Final answer:
The auditor should test from the meter department records to the sales register, ensuring that all customers' electricity consumption is billed by checking if meter readings correspond with billing records.
Step-by-step explanation:
To determine whether all customers of an electric utility company are being billed, the auditor should test from the meter department records to the sales register. This is because the meter department records will have the actual readings of electricity consumption, which is the basis for generating bills. By comparing these records to the sales register, the auditor will be able to verify if all meter readings have been accounted for in the sales ledger, confirming that all customers have been billed.