Measure its speed at 12 V. This speed is the speed required to output 12 V when used as a generator.
But this is only the no-load voltage speed. If you are carrying a heavier load, i.e. when the output current is larger, a higher speed is required to meet the requirements.
Generally speaking, this speed requires several thousand revolutions, so the first generator needs an acceleration mechanism, for example: a large pulley drives a small pulley. As long as it's 10:1 or higher.