You just made me realise why Google Glasses et al are such potential money makers.

Because unknown parties will see whatever you see?

They can verify that you saw their ad/content.

Google Analytics can track pages and average time spent on page. It can even track visitors flow.

Everyone should get an analytics account and see the insane amount of things that Google can track for YOU, for FREE with just three lines of extra code on your page.

It ONLY tracks the latter if they click on another page. A huge difference.

I actively block GA for that reason.

Nobody knows if you're actually reading the words, but based upon whether you scroll through the entire content, and possibly factoring in how quickly you scrolled, they could have an educated guess. If you didn't scroll at all, you probably read the first paragraph, if that. If you scrolled quickly to the bottom, you probably skim-read. If you scrolled to the bottom with a lot of pauses and it took minutes, you probably read the whole thing.

There are lots of ways of structuring the code, but usually by checking scrolldepth.

If you want to add this to your own site, checkout jQuery ScrollDepth: https://github.com/robflaherty/jquery-scrolldepth

They should put an innocuous call to action at the very end of the story to help track this.