Cal. Evidence Code - EVID § 870. Section 870.
A witness may state his opinion as to the sanity of a person when:
(a)
The witness is an intimate acquaintance of the person whose sanity is in question;
(b)
The witness was a subscribing witness to a writing, the validity of which is in dispute, signed by the person whose sanity is in question and the opinion relates to the sanity of such person at the time the writing was signed; or
(c)
The witness is qualified under Section 800 or 801 to testify in the form of an opinion.