It depends on which switch you are trying to diagnose. The brake light switch
actuated by your hand brake lever is a tiny momentary switch. If you listen carefully, you can hear it “click” when you apply your hand brake. If your brake light is not coming on, and you do not hear that “click”, it is a good indicator that your switch may be bad. Rear brake pedals typically use plunger mechanical switches. If your brake light is not working when you apply your rear brake pedal, locate the wires to the rear brake switch. Turn off the ignition. Unplug the two wires and jump the circuit with a small piece of wire. If you turn on the ignition and the rear brake light is on, your switch is bad.