20 CFR § 404.726 - Evidence of common-law marriage.
(a) General. A common-law marriage is one considered valid under certain State laws even though there was no formal ceremony. It is a marriage between two persons free to marry, who consider themselves married, live together as man and wife, and, in some States, meet certain other requirements. We will ask for the evidence described in this section.
(b) Preferred evidence. Preferred evidence of a common-law marriage is -
(1) If both the husband and wife are alive, their signed statements and those of two blood relatives;
(2) If either the husband or wife is dead, the signed statements of the one who is alive and those of two blood relatives of the deceased person; or
(3) If both the husband and wife are dead, the signed statements of one blood relative of each;
All signed statements should show why the signer believes there was a marriage between the two persons. If a written statement cannot be gotten from a blood relative, one from another person can be used instead.
(c) Other evidence of common-law marriage. If you cannot get preferred evidence of a common-law marriage, we will ask you to explain why and to give us other convincing evidence of the marriage. We may not ask you for statements from a blood relative or other person if we believe other evidence presented to us proves the common-law marriage.