A typical case will involve a family member contacting me to complain about the dispositions in a will or trust of a deceased relative, with allegations that another family member or a caregiver "got" to the deceased relative to unduly benefit themselves at the expense of the other family members.
To properly analyze the case, I obtain copies of all prior testamentary documents (to determine if the terms are at variance with the current documents, and how great a variance there is).
I will also obtain all of the available medical records for that deceased relative, for the relevant time period. That includes hospital records, nursing records, physician records, records of mental health professionals, and any other medically related records that can provide insight as to the physical and mental state of the deceased relative at the time the will or trust was executed by them.
I don't pretend to be a medical professional. Rather, I rely on the services of a forensic psychiatrist to assist me in the analysis, and to suggest other areas of discovery that may be useful in assisting that forensic psychiatrist in reaching an opinion regarding both the susceptibility of the deceased relative to undue influence and the actual use of undue influence in procuring the contested will or trust. This is also true when the capacity of the deceased relative to engage in a testamentary act is called into question (capacity and undue influence being regular bedfellows).
To the extent necessary, I will take depositions of the family members or bad actors in question, as well as of physicians involved in the direct treatment of the deceased relative (assuming that the deceased relative was under doctor's care).
On occasion, the bad actor gives in, withdraws from consideration the trust or will in question, and the matter is settled.... on other occasions, the only resolution that can be had is by trial.
I always bear in mind that everyone influences the actions of everyone else; it is only when the influence appears to rise to the level of undue influence that I have work to do.