Like computer graphics, you can only tell the obvious or bad ones.
I've never seen AI writing be totally undetectable or on its own particularly good, but if it someday gets good enough that AI writing is indistinguishable, I'd still appreciate when someone goes out of their way not to use it.
What do you think is a more artful way of living, spitting into food undetected, or respecting the choices and boundaries of other people?