Understanding, as it is relevant to the real world, can be accurately measured by performance on tasks.

If I ask you to design a more efficient airplane wing, and you do, why would I have any reason to say you don't understand airplane wings?

Maybe you don't have perfect understanding, and maybe we understand it in different ways.

But to do a task successfully at a high rate, you would have to have some kind of mental/neural/mathematical model internal to your mind that can predict the outcome based on changing inputs in a way that is useful.

That's understanding.