20 CFR §219.31

1. (a)Preferred evidence. Preferred evidence of a ceremonial marriage is—
   1. (1)A copy of the public record of the marriage, certified by the custodian of the record or by a Board employee;
   2. (2)A copy of a church record of the marriage certified by the custodian of the record or by a Board employee; or
   3. (3)The original certificate of marriage.
2. (b)Other evidence of a ceremonial marriage. If preferred evidence of a ceremonial marriage cannot be obtained, the applicant must state the reason therefor in writing and submit either—
   1. (1)A sworn statement of the clergyman or official who performed the marriage ceremony; or
   2. (2)Other convincing evidence, such as the sworn statements of at least two persons who have direct knowledge of the marriage, preferably eyewitnesses to the marriage ceremony.