Before testing the condensor fan, unplug the refrigerator to avoid an electrical shock hazard.
Clean the fan. If the fan blade is damaged, it can be removed from the fan motor and it should be replaced.
Spin the fan blade, it should move smoothly and freely. If the fan does not move smoothly, replace the fan motor.
If the fan motor cannot be accessed for testing, remove the motor now. The fan motor is held in place with screws and possibly a mounting bracket. Remove the bracket and the screws and pull the motor back away from the housing.