California Code, Public Utilities Code - PUC § 22745

Any person offering to vote may be challenged by any elector of the district.  In the case of a challenge the judge or clerks of the election shall administer to the person challenged an oath substantially as follows:  “You do swear that you are a citizen of the United States, that you are 21 years of age, that you have resided in this State one year, in this county 90 days, and in this airport district 30 days preceding this election, and that your name is on the great register of this county and was on the great register of a precinct in this airport district at least 30 days before this election, and that you have not before voted this day.”  If the person challenged takes this oath his vote shall be received.  Otherwise his vote shall be rejected.