Imagine you have a motor with unknown specs and you want to know the RPM it has.
 
 A very easy way to find it out is to glue a piece of tape onto the axel like in the picture. Take a piezo element (like the once you find in the digital watches) and connect it to an oscilloscope. 
 Now let the motor run and make sure the tape hits the piezo element as on the photo. Each time the piezo element is hit, a small bit of energy is generated and displayed on the scope window (see photo). Now simple count the number of periods between 2 hits and you can calculate the RPMs.
Have fun.