20 CFR § 219.39 - Evidence of relationship by legal adoption - parent or child.
(a) Preferred evidence. Preferred evidence of legal adoption is -
(1) A copy of the decree or order of adoption, certified by the custodian of the record;
(2) A photocopy of the decree or order of adoption; or
(3) If the widow or widower adopted the child after the employee's death, the evidence described in paragraph (a)(1) or (2) of this section; the widow's or widower's statement as to whether the child was living in the same household with the employee when the employee died (see §§ 219.50 and 219.51); what support, if any, the child was getting from another person or organization; and if the widow or widower had a deemed valid marriage with the employee, evidence of that marriage (see § 219.33).
(b) Other evidence of legal adoption. In some states the record of adoption proceedings is sealed and cannot be obtained without a court order. In this event, the Board will accept as proof of adoption an official notice received by the adopting parents at the time of adoption that the adoption has been completed or a birth certificate issued as a result of the adoption proceeding.