The definition of forensic psychology, often referred to as forensic psychiatry -is 'psychology used in, or connected with Courts of Law.'
Thus pleadings of guilt or innocence on the grounds of insanity, - in no sense - require the presence of an expert witness in The Court. Just as forensic medicine requires a coroner and coronial enquiry offering expert opinion on evidence relating to cause of death.
Psychological understandings can be used in evidence; for example, special or unusual needs, reactions, motives, crimes of passion and pleas of mitigation (lessening).
The FBI introduced profiling of serial killers in the 1950s-1970s after exhaustive study of what motivates these murderers. What their habits are and what drives them and what lack of social conscience-formation they may exhibit as a specific group. This type of investigative, trait-profiling psychology allows police and FBI investigators to predict a certain amount of behavior and to possibly save lives.
Information gathering is the starting point and there are now some Internet-based tools you can use, with any home or office computer, to find out information from massive United States Public Records databases:
Investigation of individuals - some tools for P.I.s and forensic psychology