Do you have the data card for your car? That is the place where you have all the numbers together, if I remember correctly (vin, chassis etc.). Last 3 or 4 digits of those numbers are stamped on various parts of the car. It is not a direct proof, but makes the car authenticity more probable. Similarly as engine number, transmission number, etc.

Of course you can compare the numbers on the data cards with a plate and see that they match the VIN number, but authorities may not plates in the car seriously. But parts of these numbers are stamped on car parts as I wrote above and this should be convincing.

Here the previous ownership related documents play a key role - you have to have them crystal clear.