20 C.F.R. § 219.30
When evidence of marriage is required
(a) When an application is filed for benefits. Documentary evidence of marriage is required when an individual files for a monthly annuity, lump-sum death payment, residual lump sum, or Medicare coverage, as the wife, husband, widow, widower, divorced spouse or surviving divorced spouse, or stepparent of the employee. A claimant may also be required to submit evidence of another person's marriage when that person's marriage is necessary to determine the applicant's entitlement to benefits under the Railroad Retirement Act.
(b) State law. In deciding whether the marriage to the employee is valid or not, in a case where the employee is living, the Board will follow the law of the state where the employee had a permanent home when the applicant filed an application; in a case where the employee is dead, the Board will follow the law of the state where the employee had a permanent home when he or she died.
(c) Types of evidence. What evidence will be required depends on whether the employee's marriage was a ceremonial marriage, a common-law marriage, or a marriage that can be deemed to be valid.