In California private employers for the most part are prohibited from asking a job applicant about an arrest which did not result in a criminal conviction.  The exception is that an employer may inquire about an arrest in a pending case.  The relevent law in the area is California Code of Regulations 7287.4(d)(1)(B) and Labor Code Section 432.7. Which provides as follows:

(1) Criminal Records. Except as otherwise provided by law (e.g., 12 U.S.C. 1829; Labor Code Section 432.7), it is unlawful for an employer or other covered entity to inquire or seek information regarding any applicant concerning:

(A) Any arrest or detention which did not result in conviction;

(B) Any conviction for which the record has been judicially ordered sealed, expunged, or statutorily eradicated (e.g., juvenile offense records sealed pursuant to Welfare and Institutions Code Section 389 and Penal Code Sections 851.7 or 1203.45); any misdemeanor conviction for which probation has been successfully completed or otherwise discharged and the case has been judicially dismissed pursuant to Penal Code Section 1203.4; or

(C) Any arrest for which a pretrial diversion program has been successfully completed pursuant to Penal Code Sections 1000.5 and 1001.5.

The law provides for damages in cases where employers violate the provisions set forth above.  Therefore, for example, if a person were arrested but all charges were later dropped in Court, he or she would fall under the protections contained in the CA regulations.