I'll go with Michael Savage's definition. A country is defined by language, borders, culture.

Presumably, if one demonstrates mastery of language(1) and cultural knowledge(2), he can be assumed to be a countryman.

1) This practice dates back to biblical times. "Shibboleth": mid 17th cent.: from Hebrew šibbōleṯ ‘ear of corn,’ used as a test of nationality by its difficult pronunciation (Judg. 12:6).

2) Common example: During WW2, if a suspected spy were encountered, mastery of language and accent may not be enough to assess his status. Cultural questions were asked: Where does the NY Mets play? What's Mickey Mouse's girlfriend's name? Questions could also be about celebrities, movie stars, music, novels, etc.