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.
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....
