Code of Civil Procedure 1856(g) provides that parol (verbal) evidence to explain the circumstances under which the agreement, to which it relates, explain an extrinsic ambiguity or otherwise interpret the terms of the agreement, or to establish illegality or fraud. Further, parol evidence is admissible even if the contract itself is clear or not in dispute.
See also recent case of IIG Wireless, Inc. v. Yi following Riverisland Cold Storage v. Fresno-Madera Production Credit Assn. (2013) 55 Cal.4th 1169 which reversed the 1935 California Supreme Court Bank of America v. Pendergrass case. CCP 1856 is the tool plaintiffs can use against fraudsters!