I'm playing through Storm King's Thunder, and came across an unusual magic item:
The Gift of Truth feature for this item says:
Whenever a creature utters a lie while within the 30-foot-radius
sphere, that creature takes 5 psychic damage and flinches visibly.
This is pretty cool, but it makes me wonder:
What spells or magic items are available to detect lies?
Best Answer
Zone of Truth
With some careful questioning, we can get the job done with zone of truth:
Clever questioning under zone of truth can lead to the target avoiding answers, which can inform you what is truth and what is lie.
Detect Thoughts
You could also give it a go with detect thoughts:
There is a lot more to the spell description, but this particular quote can give you an idea of how useful it could be for detecting truth and lie.
True Polymorph/Shapechange
We can use true polymorph to transform into a drow inquisitor (CR 14) which has the ability discern lie:
This one is possibly the most reliable on the list, but has the obvious drawback of requiring a 9th level spell. The drow inquisitor’s ability just works.
Planetars also make suitable options for true polymorph with their divine awareness ability:
Additionally, the planetar is a suitable choice for the spell shapechange, but the drow inquisitor is not.
Charm/Dominate Person/Monster
The spells charm person and dominate person can assist in getting a target to tell you the truth, though they do not techincally detect if a particular statement is true.
Charm person makes the target regard you as a friendly acquaintance, which may make them more likely to tell you some things - anything they would divulge to a friendly acqaintance.
Dominate person has a much stronger effect:
You could simply command the target to truthfully answer any and all questions you ask, and they will be forced to comply.
Be advised, these spells work only on humanoids, the more powerful variants charm monster and dominate monster will be necessary for more exotic targets.
Ring of Truth Telling
There is also the ring of truth telling:
It doesn’t confirm when someone is lying, but it helps for the insight check.
Truth Serum
There is also the truth serum which replicates the zone of truth spell:
Note, a creature must drink the entire dose, which may require some deception.
Grovelthrash
There is a sentient magic war hammer from Explorer’s Guide to Wildemount called Grovelthrash which has this ability:
Not a sure thing, works like ring of truth telling.
Honorable Mention: Ear for deceit.
The inquisitive rogue has an ability called Ear for Deceit:
Honorable Mention: Mantle of Whispers
A more roundabout and violent way of determining if a humanoid was telling the truth is the college of whispers bard ability Mantle of Whispers:
This method requires you to kill a humanoid to learn information, so will often not be a great method. But it works in a pinch, and in the pinchiest of pinches you could resurrect the target with magic.
Honorable Mention: Philter of Love
It might be a long shot, but if you could get a target to fall in love with you with the philter of love, they may be more likely to tell the truth to you.