Isaac Burk was Lithuanian, Henrietta Mahler was Latvian

How do I know that my grandfather Isaac Burk was Lithuanian and his wife Henrietta Mahler Burk was Latvian? How do I know for sure that he appears at left in this photo (with my grandmother Henrietta Mahler Burk at right, taken at the wedding of their youngest daughter, in center)?

Because his Declaration of Intention to apply for citizenship has his nationality AND a photo! It also has his terrible signature--clearly writing English was still a struggle, after all his years away from his homeland.

Isaac came from "Kovna" according to these documents. He and Henrietta married in New York City, but where, when, and how they met is a mystery (at this point). More research is in my future. UPDATED 2022: This is definitely Isaac, with his younger daughter Miriam, her new husband David, and Isaac's wife Henrietta. Photo from 1937.