noun
BAD : It's hard to tell which nation he comes from.
GOOD : It's hard to tell which country he comes from.
◆◆◆
A person comes from, lives in, feels part of, etc a particular country (NOT nation ): 'Some people in this country think that the leadership is too weak.' 'People living in former Soviet bloc countries are undergoing a difficult period of transition.'
Nation is less common than country and is mainly used when a country is considered as a political or economic structure: 'Japan has become the richest nation in the world.' 'Representatives from the world's leading industrial nations will meet next month in Geneva.'
Note the alternative: 'I couldn't tell his nationality.'