One way is to time one revolution and take the reciprocal. I never figured out a feasible way to do this.
I ended up painting black stripes all around a cassette flywheel to gauge its speed variations.
The straightforward way is to count revolutions during a one second window. This will give you a usable value.
Reset the counter every second.
Or paint two stripes. Or use a 1/2 second window. Etc.
So that you get a usable figure.
You can count digitally.
Or you can use a charge bucket circuit whose output voltage is proportional to the frequency.