I've been out of the space for about a decade, so I can't say how things have continued to evolve, but the only consistent antidote to fraud has been a trusted third party.

I spent a lot of my time back then pulling in metrics from third parties like Nielsen (who do traditional TV viewership numbers) as well as companies who specialized in "viewability". We could include that data in our real-time auction and bidders would adjust what they were willing to pay based on the reputation of the ad opportunity. At least that's the theory. In practice there was so much indirection and reselling that it was hard to say what was true.

As far as the technical answer: it's a game of cat-and-mouse but we found some interesting metrics we could dig in on. One was that fps of a video hidden behind another element on the page would have a drastically different rendered fps than one in the foreground. That would be different between browsers (and browser versions) but the viewability vendors would find all those quirks, measure, and analyze to give us a normalized score. For a hefty fee, of course.

I worked at a company that did this.

Here is the formula for viewability:

Percent In View = Area of the Intersection of the Ad and the Viewport / Area of the Ad

You would get the area of the ad using getBoundingClientRect, and the area of the viewport using window.innerWidth and window.innerHeight.

It was not possible to do this if the ad was within a hostile iframe (cross origin iframe) so you needed to use a third party source for this information like SafeFrame.

All of this was greatly simplified when Intersection Observer was officially supported by modern browsers.

> I imagine it has to be somewhat "bulletproof" since money is involved.

The GP comment pretty much shows how bulletproof it wasn't, it isn't, it never will be. You also have bad actors in the space that will load ads under other ads so that the same page load continues to generate impressions. The space behaves as if there is no incentive to be honest.