What can a voltmeter show about a hidden switch?

A voltmeter can indicate whether a hidden switch is open or closed by measuring the voltage across its terminals. If the switch is closed, the voltmeter will typically show a voltage reading consistent with the circuit's supply voltage. If the switch is open, the voltmeter will show no voltage or a significantly lower reading, indicating a break in the circuit. This allows for the identification of the switch's status without direct access.