A common misconception about law enforcement. A drug dealer or prostitute is chatting with a potential customer. But before they can actually go through with the deal, the pusher/hooker asks a simple question.
"Are you a cop?"
According to pop culture, an undercover cop has to answer truthfully, otherwise they're guilty of entrapment. The cop may employ careful use of Exact Words, saying something to the effect of "Do I look like a cop to you?", in an effort to dodge the question without outright lying, or in rare cases, may come right out and admit the truth.
Contrary to popular belief, this is not Truth in Television: Entrapment is specifically when a law enforcement officer induces another person into committing a crime when they would have been unlikely to do so had the cop not approached them. An officer has the complete legal right to lie about their identity if the job requires it. It isn't entrapment because whether or not the person is a cop would have no effect on a criminal's intentions to commit illegal acts, and undercover stings would be pretty useless if a cop was required to blow their cover whenever asked a single question. Ask this of a real life undercover cop, and they'll just say "no, I'm not a cop."
All that said, one of the reasons that this trope still exists in pop culture and the public consciousness is because cops are in no hurry to correct it. Letting this misinformation float around helps the police with their job, after all. If people think that an undercover cop has to say they're a cop when asked, answering with a straightforward "no" can ease the tension in an undercover sting.
See also: If You're So Evil, Eat This Kitten, where someone has a cop perform a criminal activity to prove they aren't a cop.
- In Alias, drug dealer Denny Hayes asks Jessica Jones, who's trying to infiltrate his inner circle, if she's a cop, and says that if she is, she's legally required to tell him. She isn't, but this doesn't stop him from ordering his goons to beat her up.
- Deep Cover includes a scene where the undercover cop protagonist is asked point blank if he's a cop. To cover himself and his investigation he admits that he is, but answers the question in an incredibly sarcastic way to make it seem like he's bullshitting.
- In The Departed, during the talk with his criminal cousin Sean on the balcony, Billy Costigan is asked to tell if he is a cop, which he denies. (He is, being sent undercover into special work infiltrating organized crime units. His record as a police trainee is public knowledge)
- Tom and Max go into an underground porn market and a guard at the door asks everyone who goes in if they're police officers or affiliated with the police. From his wording and intonation, it's clear he's been instructed to ask the same question to everyone, presumably under the belief that cops are required to answer.
- In an earlier scene, Max insists that Tom (actually a private detective) tell him if he's a police officer, saying that legally he's obliged to do so.
- Appears in the Myth Adventures novel M.Y.T.H. Inc in Action, when "Are you a cop?" is Guido's automatic reaction to being asked if he's with the Mob, and his narration "explains" that if the answer is yes, they have to say so.
- Breaking Bad: The season 2 episode "Better Call Saul" opens with Badger selling drugs on a park bench. A lanky looking dude named Getz walks up to him and asks if he's selling. Badger says he so smells bacon, pointing out two "inconspicuously" parked vans that must be surveillance vehicles. Getz denies that he's an undercover cop and actually considers not going through with the deal, especially when Badger makes him lift his shirt to reveal that he's not wearing a wire. After a bit of pondering, Getz then has an epiphany: if you ask a cop to identify himself as a cop, he is obligated to tell you. It's in the US Constitution. So Badger asks him if he's a cop. Getz holds up his hand, like he's swearing under oath, and says he's not a cop. Satisfied, Badger sells him a packet of meth. Getz takes the meth, then promptly whips out a gun and police badge, and arrests Badger on the spot (hilariously, the vans that Badger identified as police also pull up). Later, while Badger is being interrogated, he is still peeved at Getz because he still believes that urban legend and thinks Getz is screwing around with the Constitution.
- The Mentalist: The episode "Black Helicopters" has Jane and the team of FBI agents he and Lisbon now work with tangle with an anti-government commune in Texas. Agent Fischer investigates a farmer's market incognito after meeting with a wall of silence as an FBI agent. After one of the members she talked to previously outs her, she is irately informed that she is required to identify herself as a law enforcement officer.
- In one of the last seasons of Dexter, Hector Estrada, the last guy left from the group who killed Dexter's mother, gets paroled from prison and Dexter pretends to be a criminal and approaches Estrada so he can get close enough to kill Estrada. Estrada agrees to talk business at a later date, but first requires Dexter to loudly and clearly state that Dexter isn't a cop. Dexter, who really isn't a cop but a forensic examiner, does so without hesitation. Even if he were, it wouldn't count for much, since Dexter's intent isn't to arrest him.
- In The Meeting Place Cannot Be Changed, Sharapov, an undercover MUR agent, faces this issue when he tries to infiltrate a gang. The gang members suspect him of being a paid informant or a plant more than an undercover police officer though. He answers with a categorical denial: "Do I have to provide a document from the Militsia that states I'm not in it"?
- In That '70s Show, Hyde is selling hash brownies, and asks his customers if they are cops or in any way associated with the police before selling them anything.
- Double Subverted in an episode of The Sentinel. Ellison is working undercover in a prison suspected of corruption, when his cellmate asks him if he's a cop. Ellison looks like he's about to deny it, but the man cuts him off, saying that if he's not a cop, then they have nothing more to discuss (i.e. he hopes Ellison is a cop, so he can stop what's going on in the jail).
- One of the very first episodes of Law & Order had a prostitute ask Greevey if he's a cop and he says no, then finds cause to arrest her. In the station, she says "You said you weren't a cop". He just says "I Lied."
- Blackadder Goes Forth has a variant when Edmund is tasked to hunt a suspected mole in headquarters. Baldrick suggests asking suspects, "Are you a German spy?" Edmund naturally points out the prospect of the suspect lying.
- Played with in House: a Patient Of The Week is buying drugs while sleepwalking and House sends Thirteen and Taub to get some of the product to see if it can be the cause of the patient's condition. When they find the dealer she starts to make a run for it (since she had already seen them tail the patient earlier, which is why they found out about the drugs in the first place) but Taub attempts to defuse the situation:
Taub: Relax, I'm not a cop! Cops aren't allowed to say that, right?
Dealer: [stopping] ...they are, but if you were a cop you'd know that.
- The Big Bang Theory: On a trip to Las Vegas, Leonard and Raj hire a hooker for Howard to help him get over a breakup. When they ask her if she is in fact a prostitute, she immediately asks if they are cops. They say no, and she casually confirms that she is a prostitute.
- Subverted on Scandal as a flashback shows David Rosen in his prosecutor days, meeting a prostitute complaining about an arrest. She cites the trope with David dryly noting "that's not really a thing."
- When talking to prospective costumers in Monster Breeder, the player can ask them if they are a cop, since their monster breeding business is illegal (at least until they buy an official monster breeder license from the game's shop), and there is a chance the customer is an investigator trying to pull off a sting. It actually works in scaring off any snooping cops, though it runs the risk of also occasionally scaring off genuine customers.
- In Unavowed, the Chinatown greengrocer, Doug Wang, who sells an illegal sleeping aid under the table, gets it wrong in the usual way when the Player Character starts pressing him about said illegal sleeping aid. If Vicki, an actual cop, is present, she rolls her eyes and sighs in exasperation when Doug asks if it's a real rule, and says that it doesn't matter because she is currently on involuntary leave from the force.
- In Red vs. Blue, when Sister first meets Wash, she asks him if he's a cop, and insists that he has to tell her.
Wash: I'm not the police. And that's not a real law.
Sister: Well, if you're not a cop, then how do you know it's not a real law? Busted!
- Parodied in the Title Text to Questionable Content #3243.
If you ask someone if they're a murderer, they have to tell you
- Discussed in Leftover Soup, and established as not true. Cheryl does point out that undercover cops are not allowed to attempt to coerce anyone into illegal acts, which includes active soliciting. Later, Jamie is forced to prove to some criminals that he's not a cop, and resorts to (very awkwardly) offer sexual services for money.
- Futurama: When Fry's nose is stolen as an aphrodisiac, he, Leela and Bender go to a "human horn" dealer in search of it. Before letting them in, he asks if they're cops.
Leela: No, in fact, he's a crook. (points to Bender)
Bender: Stolen Pez anyone?
- In the The Boondocks episode "Guess Hoe's Coming to Dinner", Huey tries to get his grandfather to see that his new girlfriend Cristal is actually a prostitute. One question Huey asks is if Cristal is constantly asking Robert if he's a cop. She then walks by the two and tell Robert that he still hasn't answered that exact question, even mentioning the supposed "entrapment" part.