If your power supply has an on/off switch, turn the power supply on. Switch your multimeter to the range that is appropriate for reading the voltage from your power supply.

Using the multimeter at the one box will allow you to discover which of the wires is the continuous hot wire that has power when the circuit breaker is on. Mark this wire once you determine this.

Additional Tips Using a Multimeter: If you have a multimeter, you can also use it to check the voltage of a USB port by connecting it to the external pins of the USB-A connector.