Don't know the exact answer Andy, but I would have thought there are chassis/engine numbers stamp on blocks, vin plates etc. From there I guess the answer would be from the manufacturer.
I know my old mate Mr C (Greg will know who I mean) once told me motors under investigation could be identified by components labels with the serial numbers, i.e. light components etc. By under investigation I would have thought cut and shuts and stolen/cloned etc.