Which device should a high voltage detector be checked on before measuring unknown voltage?

A high voltage detector should be checked on a piezo verifier before measuring unknown voltage because a piezo verifier is designed to test the detector's functionality and accuracy in a controlled manner. This device provides a known voltage, which allows the operator to confirm that the high voltage detector is operating correctly and can accurately detect the presence of voltage. Using a piezo verifier ensures that the detector is in proper working condition, thus promoting safety and reliability during actual voltage measurements.

The other options do not serve the same purpose. For instance, an energized fault or high voltage source may pose safety hazards and does not provide a safe environment to test the detector's performance. A visual verifier does not generate a reliable voltage signal for testing the detector's response. Therefore, using a piezo verifier ensures that the high voltage detector is adequately checked in a safe and appropriate manner before proceeding with any voltage measurements.