Autonomy is by far the clearest marker of a veteran engineer.
In a new environment, can you figure things out? When given a new task, can you identify the parts of the code that need to change? Given a generic description of a task, can you work out the details?
Experienced developers generate so much value because they can work autonomously.
That doesn’t mean you should go off and work on your own, exclusively! The best developers are great communicators and collaborators. But when it comes to the details of the work — the implementation — senior developers can figure things out.