My personal sign is when you start seeing awkward collaborations start cropping up. One time when I was thrifting, I picked up a graphic novel that had the Justice League, team with the Power Rangers of all things. I glimpsed into what the plot was about out of morbid curiosity and it was just a plain generic time and dimension thing.
Nothing ever connected between the teams at all. DC Comics, while fledgling at times with how they go about their series and movies, still have far more relevance than Power Rangers do. I think the Power Rangers are just grasping at straws to keep being relevant when people have largely moved on from them.
It’s less of an issue in comedies, but main characters becoming Flanderised in drama series is where it becomes obvious they’ve run out of ideas.
For example, at the beginning of Stranger Things, Hopper had basically given up on life, and over the course of the first two seasons he finds purpose again through helping find Will, and later, raising Eleven as a surrogate daughter… And then in season 3 he becomes ANGERY MAN WHO FIGHTS PEOPLE - and that’s about it.
It runs in parallel with a show getting too many characters to handle. It accelerates the Flanderization of characters who don’t have a lot to do. Stranger Things had that problem as well, with a far too bloated main cast by the end.