The bulbs have two filaments and the sockets two contacts fed by separate wires from the lights and turn signal circuits. The socket, with wires and bulb removed can be checked with an ohmmeter looking for a low/zero ohm resistance between between +12v contacts and socket ground.

After testing that one "suspect socket" socket, per bigchief's advice, check the circuit from the main connectors to isolate front verses back verses the switch.

Note that there is also one additional bulb fed off the rear +12v run, the license plate light.