[PointerReaper]

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.

[hartator]

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.

[scholia]

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....

[hartator]

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!)