[MichaelBurjack]

Advertisers have many tools to measure "ghost ads" aka "people who haven't seen an ad but would otherwise have qualified for all your target criteria". See https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2620078 as an example.

[viraptor]

That's of course possible to measure in theory. I meant it as a response to "Facebook, Google and others make these tests possible in their platforms" since I haven't seen it exposed as an explicit option on either.

[MichaelBurjack]

Then you haven't been talking to the right ad sales teams. Google and Facebook have entire product options built around this specific concept. You can run a "ghost ads" test on Google for as little as $20k in ad spend; it's relatively accessible even for smaller players.

[jsonne]

For Facebook, Reach ads have a brand lift objective that will estimate for you. If you spend large amounts they have a measurement team that will run the actual study for you. Source: I'm running one right now for a CPG brand.

[jefftk]

You can run this yourself without any help from them: show one group ads for your site, and show another group white rectangles. Compare performance.