Deception is prominent in DnD. Whether you’re playing an intriguing campaign, trying to pull a fast one on the BBEG, or simply trying to pull a joke on your allies. And when there’s deception, there’s not always a Zone of Truth spell ready to go. Enter Insight!
Insight by its definition is the ability to gain a deeper understanding of someone or something. Whether it’s determining whether a creature was lying, or trying to understand the actions of someone, a good Insight has got you covered. Passively, insight can let you know if someone “feels off” informing you that trusting them may not be the best option (Your DM needs to learn passive checks first though). Most of the time, while conversing with a creature, someone will want to perform an Insight check to determine how truthful someone is being. And when this occurs, you roll Insight, and the creature you’re challenging rolls Deception. If you win, they tell you if they’re lying, and if you lose, you learn nothing, apart from the fact that they’re hard to read (Or they could tell you a false read if your roll is high enough).
In the end, Insight is a powerful tool for anybody who wants to be a living lie detector. And if you’re dedicated to being the best lie detector ever, a wise rogue (or a wise cleric with the Prodigy Feat) is nearly impossible to fool. Throw some perception on top of that and nobody will ever escape your gaze, and no lie will go unnoticed. Even Glibness -an 8th level spell- becomes iffy at best. Only the best liars can overcome your Insight, unless you roll a Nat 20, in which case, you tie, and you still win because that’s how DCs work! Now go catch that paladin! He’s probably grabbing a cookie from the cookie jar. You’ll know.