Where I typically notice it, is that the text starts repeating a few handful of points.

The prompt will have been to write a story on those points, and because it doesn't have much else to go off of, it will just shoehorn those exact points again and again.

I expect this to always be a telltale sign, because if your point can be made in the length of the prompt, there's a rather limited amount of noise it can add to that before it would have to go off-script.