↵how did Sadao and Hana come to know that the man was an American prisoner of war and n enemy?      

The man who had washed up was a white man and Sadao commented that there was something about him which made him seem American. It was his battered cap with the faint lettering that served as the evidence for the man's identity, confirming him as someone who had come from the American warship. It spelled out 'U.S.Navy' which was a marker of his national identity and it confirmed that he was a prisoner of war because he had escaped and had a wound in the back to show for it.