Here is a link to a blog discussing aspects of will or trust undue influence in California, Click Here.  Undue influence is most often established by circumstantial evidence, and can be difficult to prove.  The basic claim is that the person would not have voluntarily acted as he or she did if he or she had not been improperly influenced.  Not all influence is wrongful.  So . . . you need witnesses and documentary evidence to establish that the person was wrongfully influenced, or at least witnesses who will testify that the terms in the will or trust are not what the person would have voluntarily wanted.  The standard burden of proof is on the person who is trying to establish that there was improper influence.  However, in California there are now several case law and statutory arguments available that may allow the person who is alleging that there was undue influence to shift the burden of proof to the alleged wrongdoer who then must prove that there was no undue influence.  Shifting the burden of proof can be very helpful; however, it is my experience that the court will still initially require the person alleging undue influence to first establish his or her case.  I am currently involved in a will dispute case in Santa Clara County alleging issues of undue influence, fraud, mutual wills and statutorily prohibited transferee.