DETECT EVIL costs: detection spells are extremely useful, especially since the description explicitly states the spell works on a visual level. It may be that to detect the presence of evil, the caster needs something of the same kind - a shard of a cursed sword, the mummified finger of an evil wizard, the last coin of a starving family paid to the sheriff as a bribe. Another way to do it would be the spell requires a small cruelty to act as a baseline of evil detection. Finally, it could be that the ability to detect evil requires exceptional morality; a list of three behaviors in which the caster must either always perform, or never perform. These things should be hard; not like “the caster must always fight bade guys”, but more like, “the caster must never speak an untrue word”, or “the caster must give away half their wealth.”
DETECT EVIL taboos: so, the most logical taboo for me fits in the “you can’t see the forest from the trees” paradigm. Basically, if the caster is already surrounded by evil, then detecting a specific evil within it becomes difficult. Other taboos might be more based on how the GM or the caster defines “evil”. The definition of “evil” has been a difficult philosophical question for centuries, but I would advise against allowing the spell to serve as a proxy for identifying intent. The spell can detect the curse on the pile of gold, and on the death knight, but not the assassin. Now, it would detect a magical corruption which is making the king behave evilly, but not a “natural” evil. Would it detect the site of a massacre or death camp, even after those events are long past? That’ll be up to each table to decide.