Based on end-user experience during accessibility testing, I'm pretty sure they do detect the presence of tools such as AdBlock, test to see if certain elements were successfully provided before continuing with a specific set of ads, and also make some assumptions about providing "errors" to end users that only people with something like AdBlock or NoScript _should_ see.

They can't at all because nothing is loaded. Check the network tab of chrome with/without adblock enabled and you will see the difference.

Nonetheless there are websites that give you a notice to say that you have AdBlock enabled, and some even tell you to disable it in order to use the site....

Yes! But if you add the anti-Adblock script to Adblock, they won't work as well. (Doesn't mean the website will still work however!)

I've created code that test to see if ads are blocked based on failed-to-load scripts. I'm sure others have as well.

Are you a publisher or advertiser?

Neither, just a coder.