One good way is if they back up their assertions and explain their reasoning, which generally people arguing for LLM inevitability don't--they make assertions like "LLMs are only going to get better".

Contrast this with someone arguing the other side of the question like Ed Zitron or David Gerard. You may or may not agree with their arguments, but they explain, in great detail, with numbers and citations, how they arrived at their conclusions. If you don't agree with those conclusions, you should be able to point at a certain part of their argument and say "this is where you went wrong."

+1, I think the mark of a good argument is to back up assertions and explain reasoning. Although I disagree, I'm seeing well-reasoned arguments in this thread both for and against the claim "AI is inevitable".