Plug the red lead of the voltage tester into the red socket.
Plug the black lead into the black socket.
Touch one lead of the voltage tester to one terminal of the device or into the outlet. Because you're testing voltage, you need to leave the power on.
Touch the other lead of the voltage tester to the other side of the device or slide it into the other outlet.
Look at the neon bulb. If you see it glowing, it means voltage is present across the device or electrical outlet. If your voltage tester has a digital readout, take the measurement. Standard household electrical outlets should measure as 120 volts AC.