Circumstantial evidence, mate.
Prove the person coming into the dealership is the same person posting here.
It does raise an interesting point though, in that people who reveal their numberplates online are silly.
Allegedly, insurance companies have been known to look at posts on certain very large car-related forums looking for numberplates of people modifying their cars.
And insurance companies have also been known to walk around car and bike trackdays taking notes of the numberplates.
Which is why when I do trackdays on my bike, I take it there in a van and the numberplate can stay at home.