I wouldn't be absolutely sure about this. In the end, everything on the web still boils down to (mostly) simple HTTP GET requests. If you open a webpage, then you are served the file you requested (usually HTML with CSS for styling and JavaScript for special actions) and your browser handles the display of them and the execution of their scripts. This means that you can program a browser to detect and remove ads directly from the code and also eradicate malicious detection scripts potentially employed by Google that are meant to find out whether the ads are displaying correctly. If Google would want to circumvent this, they would either have to make YouTube available solely over their own app or block such behaviour on the client's end, for example by manipulating the browser's code to block ad-blocking functionality. Google is actually pursuing the latter with their Chromium browser, which is also the foundation for some others, including Microsoft Edge. This is why it's important that people start to move away and use Firefox for browsing, THE free/libre software non-profit web solution since decades. Because then Google is essentially powerless, if they don't want to take YouTube off the web.

Watching all this from the sidelines, I'm very pleased that I took the time to de-Google my critical daily services, already.

Yep, they are ramping up to disable all of the scripts and extensions.

Lately, I've been getting 403 errors in Newpipe after a video has been playing for about a minute. I think they're starting.

They are capable of detecting it because they aren't putting much effort into being undetectable. If there was a need, uBlock Origin itself could be made entirely undetectable.

Of course the YouTube script running in your browser will be able to detect changes made to the page and request blocking. However, the said script can be modified by a different extension to either receive incorrect data about blocked requests and page information, or to send a fabricated result back to the server. Google can react to it by modifying the script, and the extension would need to adapt accordingly. It's a game of cat and mouse.

If there was a need, we could have YouTube running in an entirely clean headless browser with no adblockers, while the real browser we use pulls data from it and strips out the ads.

Ultimately, currently we have the last word on what happens on our end. Unfortunately, Google's webDRM, pushed by traitors to humanity Ben Wiser, Borbala Benko, Philipp Pfeiffenberge and Sergey Kataev, is trying to change that.

Oh, they absolutely are capable of telling if you have uBlock Origin installed. However, uBlock is also capable of blocking scripts, so you can make a filter to block whatever part of the scripts on the page it is that detect your adblocker. I've never seen an anti-adblocker that didn't use Javascript, and the great thing about Javascript is that your browser can just... Ignore it.