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.
Normally when I see that, it is a signal to me that the show as intended ended but it was so popular/lucrative that moneypeople demanded it keep going, so the writers have to take an already concluded story and and un-conclude it. I’m sure shows in this situation have worked, but I’m struggling to think of one.
I suppose certain animes, especially shonen essentially do this, but they are designed from the outset to be nearly endless if successful. I’m thinking about shows like Stranger Things which clearly had one intended season, and then four seasons of whipping together something to put on screen.
Like I disclaimed at the top, it is contextual to the type of show, but I get a Spidey-sense when a show essentially restarts. Even Stargate SG-1 did it near the end, and it was overall a pretty weak few seasons.