If you are determining it before you start reading, you are doing so on the basis of reading that others have done, and your trust in their judgment. Going by the source is not infallible, take the Ars Technica scandal as an example, where an AI hallucinated quote was falsely attributed to someone. As for Lemmy, there are many articles from lesser known sources that get positive attention here that seem to use AI but do not get removed, I think mostly because they are delivering a political message that is well regarded. To identify them and have them removed without cutting out all blog content (which given the above example clearly is not even enough) requires someone to read them, evaluate them, and make the case that they should be removed, and that case has to be strong enough to overcome pushback from people biased to believe they are authentic or maybe even suspicious that the real reason for removal could be political.
It is a nontrivial task, but it's one that anyone could contribute to by developing their own sense of what is and isn't AI output, and reading far enough to make their own judgment. It's important to be able to do that without a full read, because the main threat of AI content is unlimited scaling.
any automated filtering system is beatable
What I'm calling for is not an automated system, but for people to develop skill at manually and dynamically identifying and signalling humanity in a way that resists automated systems. Attempting to do this is not inauthentic, just like trying to write poems in defined styles is not inauthentic.