It depends. Generally the actual difference is in the motor's power and therefore the maximum possible speed, but also the illegal ones do tend to be noticeably larger even if they don't strictly have to be. If I were writing the legislation the 'soft' criteria would be maximum speed and tire width.
In the Dutch context enforcement (where it is done, some work needs to happen there...) is generally done with speed measurement radar guns and what are essentially mobile treadmills where they can just floor it and see how fast it goes.
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