How to assess psychopaths objectively?
It's always difficult to be entirely objective when it comes to the mind and mental disorders. But in psychopathy one way is to use questionnaires or a checklist, one of the more famous ones is Hare's psychopathy checklist.
I managed to find Hare's psychopathy checklist revised:
You can access the checklist here for the list of statements to assess psychopathy:
Some main themes include things like:
Lack of Empathy
Using charm or manipulation
Checklists like this are an attempt to take common characteristics known to be associated with psychopathy and detect them in other individuals such as prison populations.
Thus in this fashion, objectivity is sought whilst understanding the limits of human judgment.