How do I know if my compressor is on?

You can tell if you compressor is cycling on and off by looking at the clutch (or belt pulley) at the front of the compressor. The center of this clutch (see below image) should be spinning when the compressor cycles on. Please note that the pulley itself will always spin while the center of the clutch may not. Below is a video of a compressor cycling on and off properly: