How to measure the RPM's of an optical drive? (if possible: in real time)

I can think of several ways such as opening the top and using a visual RPM measurement sensor.

The sound of the optical drive could also be picked up accoustically, put recorded audio near drive into Audacity, then one could count the spikes per second. But that is a bit more work.

What are other ways you can think of?

Maybe a magnetic sensor below the spindle? Does that even work?