basic nutshell
you attach a small interrupter to the wheel (lets be proper, it is a motorcycle, maybe a magnet system, maybe optical disk) and a sensor to a stationary place
pipe the rough signal up, make it safe, clean it, and then read it
How you go about it depends on a ton of different variables, starting with what are you attaching it to, ie a 50cc scooter, or a 1500cc turbo crotchrocket?