First, use a non-contact voltage tester. - If the tester light is ON, the wire is hot. - If the light is not ON, the wire is neutral or ground. If needed, you can use a digital multimeter. Set the multimeter to voltage mode and take the reading of each wire. The wire that shows the reading close to 120V is the hot wire. If the multimeter doesn't show any reading, that wire is the neutral wire.