Cal. Code of Civil Procedure § 417.20

Proof that a summons was served on a person outside this state shall be made: (a) If served in a manner specified in a statute of this state, as prescribed by Section 417.10, and if service is made by mail pursuant to Section 415.40, proof of service shall include evidence satisfactory to the court establishing actual delivery to the person to be served, by a signed return receipt or other evidence; (b) In the manner prescribed by the court order pursuant to which the service is made; (c) Subject to any additional requirements that may be imposed by the court in which the action is pending, in the manner prescribed by the law of the place where the person is served for proof of service in an action in its courts of general jurisdiction; or (d) By the written admission of the party. (e) If served by posting pursuant to Section 415.45, by the affidavit of the person who posted the premises, showing the time and place of posting, and an affidavit showing the time and place copies of the summons and of the complaint were mailed to the party to be served, if in fact mailed.