A character has some specific tell, often a particular tic which gives them away when they lie. Of course, another character is bound to pick up on this.
The Trope Namer, of course, is Pinocchio's nose, which increased in size whenever he lied.
The first step to becoming a Consummate Liar is to make sure you don't have any of these. These are impossible to hide from a Living Lie Detector. This is how You Can Always Tell a Liar. Not to be confused with a Gag Nose.
See also The Tell for characters with a tell that points to their emotional state rather than their honesty.
No relation to Pinocchio Syndrome.
Examples
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Anime and Manga
In Chrono Crusade, Chrono realizes something's wrong when Rosette is blackmailed by the villain to go off by herself and follows her. When she asks how he knew something was wrong, he responds "When you lie, you start babbling. I've seen you do it for four years. I know at least that much about you."
Interestingly enough, this trope comes into play subtly again towards the end of the manga. When Chrono is trying to hide an ally's death that he can sense in another room, he closes his eyes and forces himself to smile, while babbling to Rosette about how they can celebrate their victory every year together (while she questions him about what's wrong). Eight chapters later, he once again closes his eyes and has a strained smile on his face when he tricks Rosette into taking his hand so he can temporarily "stop" her time and rush off to fight Aion alone.
In an episode of Digimon: Season Two, TK's brother Matt is making excuses for the current Digidestined one of their sisters, and she promptly calls bullshit, claiming people blink a lot when they lie. "Uhhhhh....*blinkblinkblinkblink*" She promises to take his excuse...for a date, much to his defeat.
Pinochimon (Puppetmon) in Digimon Xros Wars has the same quirk as the character he's named for.
Every time Monmon from Zatch Bell! lies, his face will grow longer.
One episode of Hello Kitty Fairy Tale Fantasy retells the Pinocchio story, with the same results when Pinocchio lies. There's an additional twist, however—when the fairy says she's helped Pinocchio too many times, and she's not going to bail him out anymore, her nose grows.
Cathy's mother in V. C. Andrews' Flowers in the Attic plays with her pearl necklace — or mimics playing with one if she doesn't have one on — whenever she's nervous or lying.
In one of the Princess Diaries books, Mia's grandmother points out that Mia's nostrils flare when she lies.
In Robert Sawyer's Quintaglio Ascension Trilogy, the titular tyrannosaurian race literally turned blue in the face if they tried to tell a lie. Most of them, anyway.
Carding: My dear Spelter, you blush when you inadvertently tell the truth.
Spelter: I didn't blush!
Carding: Precisely my point.
Sam Vimes actually has one in the form of another character. Captain Carrot flinches whenever he hears a direct lie, so when Vimes tries to lie to Vetinari with Carrot in the room, Vetinari just watches Carrot's face.
Live Action TV
Ned from Pushing Daisies is a terrible liar in any case (Ned: "The plane was hijacked!" Coroner: "How'd you know that?" Ned: "DNA... ish."), but also gives himself away with his Twitchy Eye and unrelated babbling. Olive's tell is that she answers questions with questions when she's hiding something or trying to lie.
Lost, "There's No Place Like Home, Part 1": Jack explains that the fact that his stitches are bleeding means his appendectomy is healing. Kate says, "You know how when most people are lying, they can't look you in the eye? You do the exact opposite."
There was an episode of Hannah Montana all about this; whenever anyone would mention their own lying tics then she would end up doing it. Giggling and sweating were two of them, but there were more.
CSI: Crime Scene Investigation example: Catherine was able to penetrate one witness' story by recalling her eye movement: "People tend to look left when remembering, right when creating." It's handy to know, but not infallibly reliable.
There was a whole Coupling ramble on this — The Prickles, where you freeze and can feel said prickles all over your face ready to burst into rampant blushing; the Blurts, where you try to cover your prickles by blurting out denials; and the Head laugh, where you realize you were speaking too quickly and need to make everything seem alright and overcompensate with the Head Laugh.
In MacGyver, Jack Dalton's left eye twitches whenever he lies. It's mentioned in his first scene with Mac.
Including in his faked funeral.
In the Red Dwarf episode "The Inquisitor", Lister points out that Kryten's leg jiggles when he lies.
A couple of episodes actually have him say "Lie Mode" before saying something untrue. It's not clear whether or not he realizes he's doing it.
The whole point to the show Lie to Me is that the main cast has been trained to search for these when suspects in crimes are being interrogated. More specifically, Lightman actually claims in one episode that his tell is that he rubs his forehead while lying. (This was a setup for him to point out another character's, which was twisting her hands.) It's unclear whether he was actually telling the truth about that.
Used in an episode of The Lone Ranger, where the ranger used a villain's pulse as a lie detector.
Brick in The Middle has a very obvious tell: he simply whispers to himself "I'm lying."
Pointing these out is a vital gameplay elements in Apollo Justice : Ace Attorney. Particularly notable is the instance in which the killer tenses his hand in such a way that his scar looks like a skull. Yikes.
In Tales of Symphonia, Colette always giggles when she lies. Lloyd notices this when she lies that she still eats, sleeps, and feels sensation when she is, in fact, turning into an angel. Lloyd even says "You always do that fake giggle when you lie!"
In Kingdom Hearts, it's been noted that whenever Axel lies or is unsure of what he's saying he breaks eye-contact.
Western Animation
On Family Guy, Peter says he knows Ted Turner isn't telling the truth when he blinks twice. He first noticed it after watching a Barbara Walters interview, when Ted said he'd be with Jane Fonda forever.
On Ben 10 Alien Force, Kevin claims that Ben's left eyelid twitches whenever he lies. Fortunately, this also applies to an Evil Twin.
On Arthur, when the title character begins fidgeting with his glasses, it's almost certain that he's lying.
The Phineas And Ferb episode "The Beak" shows Phineas scratches his left ear whenever he lies.
