Shrug. . .if you're talking about subjective concepts of "Evil" the answer is "whatever you want" and unless you're looking for "how do you do evil in your game?" there's no real answer to the question.
If the question instead is in terms of the rules, as in "What's capitol E evil in a gameworld" you're getting into rules things that tie into mechanics for which there are actual answers.
Like, "I think you're evil" falls under the "Everyone's got an opinion and a. . .ahem. . ." logic.
while "You detect as evil" gets into the concepts of actual evil. . . .
The rules actually define it, in that Evil needs to be something absolute and flagged as evil within the campaign.
Something "Of Darkness" is touched by or has it's origin in evil, but might not be definitively evil.
(Flip side logic for "Good" and "Of the light" per the rules).
It's not for every game, and getting into capitol E evil and G good implies some form of absolute morality. . .i.e. Demons are evil, no discussion, and angels are good no discussion, if you find that confusing, then evidently you're at least of darkness, if not outright evil yourself.
In more morally ambivalent games, a lot of times "Evil" is ruled as "Enemy". . . .so priests of Thor detect giants as evil, while a priest of Loki might not. . .and things get rather complicated, as they always do when morality is subjective rather than absolute.
Using Evil and Good in a game without moral absolutism in play gets very squishy, and in the end, those two terms are meaningless in any objective sense in those games. . ."you're evil because I choose to see you that way" doesn't really need rules, that's how most people behave most of the time in the real world, and it often leads to rather nasty acts in extremes.
Trying to pin any absolute that's not tied to a game mechanism. . .i.e. short of the logic of "Demons are absolutely evil because it says so on page 32" . . .is likely never ever going to result in more than a handful of posters agreeing. . .because it's gone all squishy.
