20 C.F.R. § 222.10
When determinations of relationship as wife, husband, widow or widower of employee are made
(a) The claimant's relationship as the wife or husband of an employee is determined when the claimant applies for an annuity, or when there is a claim which would include a husband or wife in the computation of the social security overall minimum provision, or when a claim is filed for a lump-sum payment. If a deemed marriage (see § 222.14) is to be determined, the husband, wife, or widow(er) must also be found to be or to have been living in the same household as the employee (see § 222.16).
(b) The claimant's relationship as the widow(er) of an employee is determined as of the date on which the employee died. If the claimant applied for a lump-sum payment as the widow(er) of the employee, one of the following determinations is made:
(1) Whether the widow(er) was living in the same household as the employee, as defined in § 222.16 of this part, at the time of the employee's death, if the claimant is applying for the 1974 Act lump-sum death payment.
(2) Whether the widow(er) was living with the employee, as defined in § 222.15 of this part, at the time of the employee's death, if the claimant is applying for the 1937 Act lump-sum death payment, annuities due but unpaid at death, the residual lump-sum payment, or a lump-sum refund payment.
(c) In order for a claimant who has applied for a monthly survivor annuity to establish a deemed marriage, the claimant must have been living in the same household as the employee at the time of the employee's death (see § 222.16).
(d) If the husband, wife, widow(er), remarried widow(er), or surviving divorced spouse of the employee is a claimant for a monthly annuity on a basis other than age or disability, a child-in-care determination is required (see §§ 222.17 and 222.18).