To check whether your baseboard heater is 120V or 240V, you will need a multimeter. Turn the circuit breaker switch, connected to the baseboard, on and touch the multimeter leads to each connected wire. The voltage of your heater will be displayed on the voltage meter. If it is between 110V and 125V, then the heater is 120V, whereas a 240V model will show voltage between 220V and 250V.