As a CNC technician, there have been a few occasions where I've wondered if a spindle or motor shaft was spinning as fast as the motor controller claimed. An old trick I learned a long time ago involved a digital guitar tuner. The trick was to hold a piece of stiff metal shim stock, around .020" or so thick, against a protrusion on the shaft. As the motor strikes the shim it should cause it to vibrate at a frequency that the tuner would turn into a note reading. If you were to look up the frequency for that particular note, some quick math would give you a reasonably accurate RPM.
The problem with this method is that most industrial machinery is not located in a concert hall. The ambient noises in a typical factory reduce even the most careful measurement right back into guesswork. So, if we generate an electrical signal instead of relying on sound, we can isolate and refine our measurement into something useful, and gain a tolerance of around 5 percent.
A small permanent magnet
Enameled magnet wire (pretty fine; mine feels around 0.010")
A digital guitar tuner with an input jack (you can use a good one - we won't be modifying it in any way)
A 1/4" male to 3.5mm female plug adapter
The patience to splice very thin, very fiddly wire