Question

how did sadao and hana come to know that the man was an american , a prisoner of war and enemy?

Answer 1

By his face resemblence and on his cap was written usa. From which they came to know that the man was american.

Answer 2

Explanation:  

Sadao and Hana saw a man lying on the sand with wounds and turned his head and saw the face. She realized that he is a white man as he is having a rough yellow beard with long uncut hair for many days and a tortured face.  

There was a bullet wound which was opened and covered with some powder. When they were searching for other things in his body, they saw a cap which had the writing “A Sailor from American warship US Navy”.  

So, they came to know that he is an American and is their enemy who should be handed over to police.