Suppose one has a terrible pain in his chest. He tried various medicines one after the other. One day he found a new medicine and he took it. He followed all the instructions and took every precaution. Finally his chest-pain vanished and never occurred again and how did he know his chest pain got vanished? He feels it. Understand it. He is aware of it. That is how Lord Buddha knew that his mid has uprooted all the defilements. The important thing is anybody can have the same experience when they themselves attain that level.