You should have a good idea. How to know if an ad reaches people had been extensively studied for a lot longer than the internet even existed. Newspapers don't have clicks, but you still need to know if you ad works. Even on the internet, a large part of the value of ads are see the ad, buy latter without clicking on the ad. We can track this: do it.
The online ad market is extremely inefficient because it has no idea how much ad spend is even reaching people.