California Civil Code Section 1198

The execution of an instrument may be established by proof of the handwriting of the party and of a subscribing witness, if there is one, in the following cases: 1. When the parties and all the subscribing witnesses are dead; or, 2. When the parties and all the subscribing witnesses are non-residents of the State; or, 3. When the place of their residence is unknown to the party desiring the proof, and cannot be ascertained by the exercise of due diligence; or, 4. When the subscribing witness conceals himself, or cannot be found by the officer by the exercise of due diligence in attempting to serve the subpoena or attachment; or, 5. In case of the continued failure or refusal of the witness to testify, for the space of one hour, after his appearance. (1199.) Section Eleven Hundred and Ninety-nine. The evidence taken under the preceding section must satisfactorily prove to the officer the following facts: One--The existence of one or more of the conditions mentioned therein; and, Two--That the witness testifying knew the person whose name purports to be subscribed to the instrument as a party, and is well acquainted with his signature, and that it is genuine; and, Three--That the witness testifying personally knew the person who subscribed the instrument as a witness, and is well acquainted with his signature, and that it is genuine; and, Four--The place of residence of the witness.