Brake Light Circuits
Brake light circuits run in parallel. In a parallel circuit, the other bulbs can still illuminate if one bulb fails. That's why when one bulb is out, the bulb and socket are usually the best places to begin diagnosis.
Brake Bulb Inspection
Most of the time, it's apparent the bulb is bad by the discoloring inside the bulb. If not, remove the bulb and test it with a multimeter set to ohms. Place the negative tip on the bulb's case and the positive tip on the contact. If the meter reads OL or infinity, the bulb is burned and needs replacement. If the bulb reads continuity or is close, the bulb is good, and the socket and circuit should be tested next.
Place the bulb back in the socket and use a pedal depressor, or have someone depress the pedal for you. Slightly wiggle the bulb around in the socket while the pedal is depressed. A loose socket is likely at fault if the bulb flickers on and off. If the bulb is still not illuminating, measure the voltage in the socket by placing the positive probe on the brass tab and the negative probe on the inside. Battery voltage should be present. If not, test the wiring, connectors, and the related circuit.