What is the best way to determine if a CPU fan is spinning properly?

What is the best way to determine if a CPU fan is spinning properly?

• Spin the blades of the fan quickly with a finger.
• Spray compressed air on the fan to make the blades spin.
• Visually inspect the fan when the power is on to ensure it is spinning.
• Listen for the sound of the fan spinning when the power is on.
  

  Explanation: Spinning the fan blades with the power off, especially by using compressed air, can damage the fan. The best way to ensure the fan is working is to visually inspect it with the power on.