Devils, demons, they are objectively evil. Evil is part of their magical make-up. Other than that, I simply use “evil” as ‘in opposed to good’. If the PCs are good, then someone who actively wishes them harm or seeks to thwart a plan befitting their attitudes are evil.

This allows more DM discretion and keeps a bit of mystery in the game. I don’t tell them that a merchant feels evil, but rather “seems to openly hostile, cruel, and out spoken against you.” for subjective, or “dark ooze seeps from his poors, providing no concealment yet unmistakably associated with the nearly unseen black flicker in his eyes.” for objective evil. Often, I do no let magic detect subjective evil, limiting cleric and paladin capabilities (only a good thing in my campaign, where Nick has mastered the art of paladin creation to an unbelievable extent).

the workaround that I have is that alignment is replaced with faith auras. basically pledging yourself to a god brands your soul, and paladins and clerics have the ability to distinguish the soul brands of various deities. therefore, because your god grants you powers, you are also granted awareness of which particular deities are rivals or enemies to your faith (which can change as part of the petty whims of your master).

so, with this, if your plays want less ambiguity, you can limit the soul-searching and just say, “your deity thinks these followers of religion Y are enemies. go and be an instrument of divine will” but then also setup the possibility for tragic betrayals and moral arguments over whether your god is right in ordering you to drown a bag of kittens.