When a judge is challenged on a motion to disqualify him from the bench, what is the test for disqualification?

When the motion is properly and timely made the challenged judge is vested with no discretion to determine the question of his actual prejudice. It is a party's (or his attorney's) belief in prejudice that is decisive and there is probably no way in which the sincerity of such belief can be determined or measured. Disqualification is mandatory. (Jacobs v. Superior Court, 53 Cal.2d 187, 1 Cal.Rptr. 9, 347 P.2d 9.)