If it looks like nothing in the previous gen could run it without compromises, then it's next-gen. I thought this was always the way we determined what looked next-gen, and what didn't. You will never know the technical specifics of a game, so it's all eye-ball test. In that regard, I like to keep it simple. Can a PS4/X1/PS4P/X1X run this PS5 or XSX game as-is? If not, then that's next-gen. It has to be more than resolution and framerate bumps, though. Those are no different than boost mode in BC. There should be some modifications to the assets and/or graphics engine that add a noticeable level of improvement.
Yeah, start simple and add a bunch of subjective bullshit to the formula. Pretty much how it works. Some will think a game is next-gen, and some won't. Fuel for dumbass console wars.