Interestingly, adblockers should be careful about doing stuff like this, as lots of sites would love a check as simple as "make a request to /twitter-sponsered-ad-paid and deny access if it fails"
If adblocking is all about user choice then it's only fair that sites have choice to serve that content too. Right now it's incredibly hard for sites to reliably know if someone is using adblock.
Attempting to purposefully disguise that usage goes beyond any accepted ethics of using adblock today.
You can do it yourself as well -- EasyList and some others have patterns that they check for on the page path and individual elements, not only known hosts.
Keyword "reliably". It sounds easy but it's not due to the all the different plugins, rules and levels of adblocking.
There are also constant updates to counter this. If you check the adblock forums, there are dozens of threads every day about how to get around checks on sites and even disable messages that specifically show up to adblock users.
Well, the other side of it is that, imho, the adblocker has no reason to purposefully expose itself. I was not commenting on the ethics of adblock, just the fact that sloppy checks and sloppy code are a bad thing. I personally don't use an adblocker, I just don't go to sites that I have had bad ad experiences on.
1. Look into the most popular filter's used by adblock extensions: https://easylist-downloads.adblockplus.org/easylist.txt
2. Choose for example a querystring rule from the beginning (from the "General advert blocking filters")
3. Append it to the url of an existing image
4. Use the onerror event to detect if it wasn't loaded