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Google admits it's making YouTube worse for ad block users
(www.theregister.com)
This is a most excellent place for technology news and articles.
To discern if an add blocker is in use you are processing information not essential to your service.
You could, eg. Not start the stream until the add is over if it wasn't blocked without violating this. In the end whether or not the user uses an add blocker is not relevant to your ability to stream a video.