Proof

How can someone prove that they understand?

They profit from that understanding and the doing.

Whether it be winning at sports gambling because you understand the variables of the players and the setting. Or winning at investing by better understanding the political, economic, and technological components at work. Competition is won by the player that better understands.

You can also prove that you understand by using the knowledge to create. Profiting from airplanes would be pretty difficult if no one understood aerodynamics.

Win as Proof.