@schratze In cases like this, where I think there's a reasonable chance that they don't know about it but they might want to, I usually just ask if they're interested in advice on getting rid of ads completely, for example. And if they say 'no', leave the topic be.
(From experience, a lot of people *don't* know about adblocking, and are happy to learn about it, as long as you don't *assume* they don't know and/or are interested, and aren't pushy about it. And asking "do you want advice" allows them to say "no" without having to provide a reason, which seems to be the main thing that matters to people.)