How do I know if my career built anything of my own?

You can design almost any mechanism, and near enough to the end you wonder whether a life spent optimizing other people's machines ever built anything that was truly yours. That is a serious question, and it deserves more than a quick reassurance. The work you did lives in things that move, hold, and last, even if your name is on none of them. But the deeper truth is that what you built of your own is not only in metal. It is in the judgment, the patience, and the care you carry, and in whoever learned the craft by watching you. Spend an evening writing down the machines and the people your work touched. You may find the record longer than it felt. The question of what you built is worth asking honestly, and the honest answer is usually more than we fear.