The first test should be an MRI of the brain along with a psychiatrist evaluating the person and then other methods of investigation that may not be traditional medical investigations. Looking at the finger length is only a possibility, you would need a very skilled palmist to show which particular lines or marks in both hands relate to the psychological profile and health issues, not to forget the domestic life of the person.
Then looking at the persons astrology chart which lays out the entire profile of a person. That requires the exact time of birth of the person to be able to give an accurate interpretation of the individual and even with these things nothing is 100% . Did you know that the star sign of Aries is regarded as the most likely of all star signs to be a psychopath?
Even here it is the placement of planets in houses that make the personality traits of an individual and then the degrees and angles when formulated inform the astrologer of the persons behaviour and mind set. Again there can be things that may not show up. So you need several things put together to make a reasonable opinion but first is a proper medical assessment.
This link here discusses the use of MRI to view the formation of the subjects brain in diagnosing abnormalities such as psychopathy. There is no cure for psychopathy either, and some of these individuals can be highly intelligent and able to evade detection.