The fan is connected by two or three wires. Label the wire placement on the fan before disconnecting the wires. The wires are connected to the fan with slip-on connectors. Firmly pull the connector off (do not pull on the wire) of the fan's terminal. 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 replaced.
Spin the fan blade, it should move smoothly and freely. If the movement is not smooth, the motor should be replaced.
Test the fan for continuity using a multitester. Set the multitester to the ohms setting X1. Place a probe on each terminal. The multitester should display a reading of zero ohms. If the fan does not pass this test, it should be replaced.