seems like the obvious answer to that is you cover a patch of road with 5mm ball bearings, and send a waymo to drive across it. if the ball bearings behave the way the simulation says they would, and the car behaves the way the simulation said it would, then you've validated your simulation.
do that for enough different scenarios, and if the model is consistently accurate across every scenario you validate, then you can start believing that it will also be accurate for the scenarios you haven't (and can't) validate.
do that for enough different scenarios, and if the model is consistently accurate across every scenario you validate, then you can start believing that it will also be accurate for the scenarios you haven't (and can't) validate.