Many potential signs:
You find yourself finding excuses to not go to work, to take days off, or just overall don't like going to work.
If you constantly feel like the smartest guy in the room - i.e. you don't anymore learn from the others. Your own progress will be stunted if you allow yourself in a situation like that.
If you're talented enough to build applications from the ground up yourself, but the company doesn't reward you highly enough - you can potentially get a good amount of shares if you join an early stage startup
If you work on the same kind of problems all the time - being an expert on one subject might be fine for some, but I generally recommend against it as it also means that when that one technology expires you will be useless
If the company seems to be falling apart - leadership, core team members are leaving or being fired
Poor team morale for an extended period of time - if nobody cares to fix the morale issues that's not a healthy sign, find a more motivated team in another company
Culture of micromanagement - being told exactly how to spend every hour of every day, and tracking your work with Scrum poker velocity etc. useless junk intended only to make old fashioned managers feel better
Having little to no ability to impact your own work - if you can't give feedback, feature suggestions, etc. or nobody listens to your ideas
If you're being paid unfairly (other people make more without a good reason) or overall not compensated well enough (other companies pay much more)
Lots of things along that theme can be good triggers to decide to leave a company.
Anyway, never just quit - start looking for other jobs, interview for as long as it takes to find the perfect next step for you, and only after you've signed the contract to start at the new place do you quit.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.