You can check for voltage to the oxygen sensor heater, and from the oxygen sensor, with an electrical multimeter. You could check the resistance of the oxygen sensor heater, also.
To see if the PCM (engine computer) is making a ground for the oxygen sensor heater circuit, you would check for a voltage drop, when the ignition switch is turned ON for a few seconds, on the heater ground wire before it goes into the PCM.
Don’t bet on the dealer’s mechanics performing these simple voltage checks.