Cal. Pen. Code § 1158
Whenever the fact of a previous conviction of another offense is charged in an accusatory pleading, and the defendant is found guilty of the offense with which he is charged, the jury, or the judge if a jury trial is waived, must unless the answer of the defendant admits such previous conviction, find whether or not he has suffered such previous conviction. The verdict or finding upon the charge of previous conviction may be: "We (or I) find the charge of previous conviction true" or "We (or I) find the charge of previous conviction not true," according as the jury or the judge find that the defendant has or has not suffered such conviction. If more than one previous conviction is charged a separate finding must be made as to each.
Ca. Pen. Code § 1158