Hall effect sensor - put a small magnet on the wheel and the sensor mounts alongside - your counting circuit will receive a "pulse" each time the magnet goes past.
Optical sensor - you arrange an infra red light emitting diode (or similar light source) beside the wheel, and at just one point on the wheel a hole is drilled through to let the light pass through the wheel itself. Then a photodiode or some other light sensor is on the other side. Again, it gives a signal each time the hole pops round.
(There are other types of sensor I guess, but these are the most common ones you'll find schematics for.)
Then you need some sort of electronic counter to count pulses, or maybe measure the time interval between each pulse, and convert that to a "speed" measurement. You don't say anything about a counter - but I'd guess one of the cheaper Arduino devices will do it. (They have a lot of circuits for this sort of thing if you dig through their website.) Here's an example to show the principle.