To be precise, by "evil" I am referring specifically to the weakness of the trait of (or compulsion) for “genuine care and concern for the wellbeing of others” with respect to the strength of the trait of (or compulsion for) "the pursuit of self-interest".
Or, more succinctly, the weakness of the force of conscience as a constraining force on the predatory compulsion of self-interest.
“The battle line between good and evil runs through the heart of every man."
Some humans seem to lack this “genuine care and concern for the wellbeing of others”
Of those who do, there seem to be at least 2 distinct groups:
1: Those who lack it permanently. (irredeemably evil)
2: Those who lack it only temporarily. (temporary insanity)
By "temporary insanity" I am referring to the phenomenon of people who seem to act without care and concern for the well being of others, but who seem to do so because they lack the ability to control their fear at the time and are later genuinely remorseful for their behavior.
By "irredeemably evil" I am referring to those who never experience any genuine remorse for such behavior.
Difficulty #1: The seeming absence of a single underlying cause
Psychopathy is probably the best example of "irredeemable" evil. There are methods like the PCL-R to detect this particular type of evil. However, there seems to be growing evidence that narcissism also has a strong heritable component. The methods for detecting psychopathy do not seem to be applicable to narcissism.
To add to the difficulty of the problem, the evidence would seem to suggest that the absence of moral constraint to predatory compulsions may not have a single cause.
For example, there is some evidence to suggest that psychopathy may be the result of weak pain signals in general.
One interpretation of this is that conscience emerges and is strengthened by the process of persistent pain giving rise to rumination. i.e. When our actions harm others, the persistent guilt that we feel, gives rise to a resolve to change our behavior both to relieve the pain in the moment and to avoid feeling this pain again in the future. However, if the pain signal is too weak to give rise to rumination, then the conscience does not form.
On the other hand, narcissists seem to typically have normal levels of pain for themselves but lack the affective empathy which would trigger similar levels of pain when other humans are suffering. One interpretation of this is that the persistent pain which gives rise to the rumination which gives rise to conscience does not arise, due not to the inability to experience pain in general, but rather to the inability to experience pain of others due to the absence of affective empathy.
Given these different ways in which the growth of conscience can be interrupted, it's possible that the most effective means of discernment between "irredeemable evil" an "temporary insanity" might be to identify the traits of those who have a fully developed conscience and look for the absence of those traits in the individual in question.
Difficulty #2: Variance in the size of the circle of genuine care and concern.
To further add to the difficulty of this problem, it seems that the size of the circle of "genuine care and concern" varies considerably amongst humans.
For some, it extends only to kin.
For others it might extend to a larger tribe.
For others still, it might, as some religions would encourage us, extend to all humans or even all living beings.
To be more precise, therefore, given that evil is the perception of the absence of "genuine care and concern for the well being of others", and that those who lack "genuine care and concern for the well being of others" will feel compelled to fake it in order to survive and thrive in society, this question seems to break down into the following 2 components:
1: pre-discovery: What is the most accurate method that potential prey can use to detect that the "care and concern" displayed by the skilled predator is, in fact, a deception?
2: post-discovery: Having witnessed evidence of the absence of "genuine care and concern for the well being of others", what is the most accurate means of determining if this is due to a permanent versus temporary inability to expand the circle of care and concern?