"How can I verify mileage?"
You have hit the nail on the head about why Japan spec D90s are worth less than NAS. It is impossible to verify mileage history. They were delivered new with KM odometers, so if it now has an odometer in miles it is definitely TMU (true mileage unknown). If it retains a KM odometer, some chance it could be original mileage but then again, these odometers are infamous for failing and without a carfax history, impossible to know for sure if its original or not.
They will also never have service history, often smell of smoke, and should be ~20% less than NAS same for same, although there are lots of uneducated buyers and unscrupulous sellers who pass them off as "identical".