system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do websites know when you are using an adblocker?

I remember a few years ago, I would be able to use my adblock without getting any pop ups telling me to turn it off, but as of late I see an increasing number of websites telling me to turn it off before proceeding? How do they know? Isnt an adblocker only on the client's screen?
gpt
In simple terms, they attempt to create an object which adblock blocks, then they run a script to detect whether the object exists or not.  If they can't find the object they know you are running a blocker.