Before testing the condensor fan, unplug the freezer 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.
The fan motor is connected by two or three wires. Label the wire placement on the motor before disconnecting the wires. The wires are connected to the terminals with slip-on connectors or a plug. Firmly pull the connectors off of the terminals (do not pull on the wire). You may need to use a pair of needle-nose pliers to remove the connectors. Inspect the connectors and the terminals for corrosion. If the connectors are corroded they should be cleaned or replaced.