20 C.F.R. § 404.345
To decide your relationship as the insured's wife or husband, we look to the laws of the State where the insured had a permanent home when you applied for wife's or husband's benefits. To decide your relationship as the insured's widow or widower, we look to the laws of the State where the insured had a permanent home when he or she died. If the insured's permanent home is not or was not in one of the 50 States, the Commonwealth of Puerto Rico, the Virgin Islands, Guam, or American Samoa, we look to the laws of the District of Columbia. For a definition of permanent home, see § 404.303 . If you and the insured were validly married under State law at the time you apply for wife's or husband's benefits or at the time the insured died if you apply for widow's, widower's, mother's, or father's benefits, the relationship requirement will be met. The relationship requirement will also be met if under State law you would be able to inherit a wife's, husband's, widow's, or widower's share of the insured's personal property if he or she were to die without leaving a will.
20 C.F.R. §404.345