We will use a multimeter and check the outlet with the help of the test leads, and the multimeter screen will tell you whether the outlet is 120 volts or 240 volts. Another way to check; If you check a socket wire and you find black and white wires, the electrical supplies are likely to have 120 volts. If your socket has black and red wires, your connection wires are likely to consist of 240 volts.