California Code, Evidence Code - EVID § 1416

(a) Having seen the supposed writer write;

(b) Having seen a writing purporting to be in the handwriting of the supposed writer and upon which the supposed writer has acted or been charged;

(c) Having received letters in the due course of mail purporting to be from the supposed writer in response to letters duly addressed and mailed by him to the supposed writer; or

(d) Any other means of obtaining personal knowledge of the handwriting of the supposed writer.