In Dungeons & Dragons Wisdom (Insight) checks commonly lead players to pile-on checks. One character talks to someone, asks to roll insight, and then everyone adds their roll. The group supposes that just one success will spot a lie. If the dungeon master allows such checks, someone almost invariably uncovers any deception. By such rules, lying to big groups becomes impossible, which makes insight checks the most unrealistic thing in a game with djinns in bottles who grant wishes.
If you want any intrigue and deception in your game, never roll group insight checks where one high roll brings success. Instead, opt for one of two methods. The choice of method depends on whether you, as DM, want players to roll their check.
If the players roll, the numbers on the dice give players unearned hints. Low numbers tell the players they probably failed and not to trust their insight; high numbers suggest they succeeded and that, for instance, an NPC who appears honest can be trusted.
If you roll in secret, the players feel deprived of some control over their fate. After all, some DMs will fudge rolls to protect a planned narrative. Also, players like rolling dice, especially if rolling gives unearned hints.
Players roll group checks
If you allow players to roll, call for a group check where everyone makes a Wisdom (Insight) check and at least half the group must succeed.
This method may see odd, because group checks apply to situations where one failure could potentially cause the whole group to fail. For instance, one noisy character could alert the guards the party wants to sneak past. But group checks actually fit insight checks with no sure answers. If at least half the group succeeds, the successful characters reveal their insight to the others. If too many characters fail, the group suffers a difference of opinion that leaves everyone uncertain. Or perhaps Terry the Apothecary just proved hard to read.
Don’t tell players which characters suspect lies. Players who know that and their die rolls gain a metagame-based lie detector.
Set the difficulty class for the checks by adding 10 to the liar’s Charisma (Deception) bonus, so the DC equals the liar’s passive deception.
DMs roll a single check
As a DM, you could roll a secret, group Wisdom (Insight) check, but tracking several die rolls and bonuses would slow the game. Instead, roll one check for the character in the scene with the highest Wisdom (Insight) bonus. By using the highest insight score rather than a group of scores, this method benefits the players. On the other hand, the players lose any hints they gain from seeing the numbers. Don’t grant advantage for help coming from the other players. We don’t want to make spotting lies unrealistically easy. This method presumes that the rest of the group offers little help to the most insightful character. Either the others also spot the deception, or they muddy the waters by being more easily fooled.
Alternately, roll one Charisma (Deception) check for the liar against a DC set by the group’s highest passive Wisdom (Insight) score. If the deceiver fails, describe signs of deception. On success, the liar seems legit. I like this reversal because the odds stay the same, but you roll on behalf of the more active character.
Usually a liar only needs to make one deception check, but if the pressure increases thanks to sharp questions, or their lies begin to unravel, you might require fast talking and another check.
Success and failure
Whatever type of check you use, if the outcome favors the players, a liar shows signs of deception and an honest character seems trustworthy. Otherwise, the target of the check seems hard to read.
Rather than flatly stating that someone lies, describe signs of deception: A lying person may sweat or otherwise appear anxious. Perhaps they start speaking in a manner that seems rehearsed. Someone with something to hide might avoid eye contact or become hesitant while speaking. Perhaps their words and body language fail to match. For example, they might nod yes during a denial. For countless more symptoms, search the internet for “signs of deception.”
When a check goes badly against the players—call it a fumble even though D&D lacks critical failures—the party may get the wrong impression. Perhaps an honest person shows misleading signs of deception. Follow what works for the story and your inclination to deceive the players. Maybe an honest person just feels nervous in the presence of such esteemed adventurers (or such temperamental and murderous treasure hunters).