Hello: For any electrical testing, the basic tool is an analog volt-ohm meter, the kind with a dial. They are cheap and readily available. There is no troubleshooting an electric heater without one. If you’re not comfortable with it, get a friend who can show you or hire an electrician and watch. When dealing with electricity, the choices are black and white 😀 Assume any wires are live until you prove otherwise, with the meter.