Be careful, you might get what you wish for. California and Oregon have been flirting with mandatory GPS trackers (that also hook to the ODB 2 port) on all vehicles that not only collect the number of miles traveled, but give a real time picture 24/7/365 where every single registered vehicle is, what speed it is going, and so on.
If it had to be by miles, the best way is an odo check every year when the vehicle is inspected with the tax paid before the inspection sticker goes on. However, common sense never seems to win. If it became a choice between helping subsidize Prius drivers versus having a ticket generated by the car itself if it goes one MPH over the speed limit or having other fallout from having the car tracked 24/7 (with that data public, as some hacker will grab it and make that database available), I'll take the EV subsidization.