Cal. Harb. and Nav. Code § 606
Proof of compliance with Section 604 shall be made in the event of service by mail by affidavit of the plaintiff or libelant or by affidavit of his attorney showing said mailing, together with the return receipt of the United States post office bearing the signature of the defendant or respondent. The affidavit and receipt shall be appended to the original summons or original citation, which shall be filed with the court from out of which the summons or citation issued within such time as the court may allow for the return of the summons or citation.
In the event of personal service outside this State, compliance with Section 604 may be proved by the return of any duly constituted public officer, qualified to serve like process of and in the state or jurisdiction where the defendant or respondent is found, showing such service to have been made. The return shall be appended to the original summons or original citation which shall be filed as aforesaid.
Ca. Harb. and Nav. Code § 606