Is there a saying or idiom for "trying to get the truth from someone by lying"?

Question

context

• Mary-Ann got home late from school. Asked where she had been, she said she had spent the afternoon at the library. Her father thinks she is lying and says: "I know you haven’t been there because I was there myself". He is lying because he thinks she will tell the truth if she realizes she has been caught.
• boyfriend – “your sister told me everything” (sister did nothing) - girlfriend – “Oh, did she? I’m gonna kill her.

The rationale for doing this is to catch someone by surprise so he will admit having lied. Soemtimes, however, the person isn’t lying and the real liar will have to reap the consequences of his reproachable action, having to admit he was just/it was just ...........(saying or idiom).....

Answer 1

Sometimes people set a trap in order to force others to make a mistake or own up to the truth. An English idiomatic phrase that fits this type of situation is to trip someone up; if the victim falls (unwittingly) into the trap, then you have successfully caught him/her out.

set a trap (for somebody)
a clever trick that is used to catch someone or to make them do or say something that they did not intend to

catch out
to show that someone has made a mistake or is not telling the truth, especially by asking them questions He asked her casual questions to see if he could catch her out.

_{Sources: Longman Dict. of Contemporary English, Macmillan Dictionary}

Answer 2

My first thought when I saw this question was that the idiom "It takes a thief to catch a thief" might be applicable. Christine Ammer, The American Heritage of Idioms (1997) explains the sense of the saying as follows:

it takes a thief to catch a thief, meaning "no one is better at finding a wrongdoer than another wrongdoer." First recorded in 1665, it remains current.

But it turns out that a longstanding English proverb is even more direct and on point. It appears in John Ray, A Collection of English Proverbs (1678):

Tell a lie and find the troth.

Wolfgang Mieder, A Dictionary of American Proverbs (1992) traces this proverb to circa 1594 in Francis Bacon, The Promus of Formularies and Elegancies, and finds recent instances of its use (with truth in place of troth) in Oklahoma and Texas.

The version of Bacon's Promus that I consulted, however, offers split sentiments on the validity of the saying, quoting one excerpt from Shakespeare opposed to it and three in favor of it. On the one hand:

To find out right with wrong—it may not be. (Richard II, i.3.)

But on the other:

I think't no sin/To cozen him that would unjustly win. (All's Well That Ends Well, iv. 2.)

It is a falsehood that she is in, which is with falsehood to be combated. (Two Noble Kinsmen, iv. 3.)

Your bait of falsehood takes a carp of truth;/And thus do we of wisdom and of reach,/With windlasses, and with assays of bias,/By indirections find directions out. (Hamlet, ii. 1.)

Answer 3

A more elaborate form of this type of situation is known as a sting operation, a setup designed to catch criminals or liars by presenting a false facade or character (in other words, catching them by lying about yourself or your actions). A sting operation is generally not created by simply verbally lying, but usually involves actions and often more than one person. But the term could be used in the situations you describe, though it would imply more preparation or planning by the interrogator than would typically be assumed:

Mary-Ann's father had set up a sting operation to see if she'd been telling the truth about her frequent late nights, telling her that he had been at the library and had not seen her there. He then faked a cell phone call to the librarian to watch his daughter's reaction.

Answer 4

The term "fishing" is appropriate here, as is "gaslighting." "Gaslighting" carries connotations of abuse, while "fishing" is a bit neutral.

gaslighting: Gaslighting or gas-lighting is a form of mental abuse in which information is twisted/spun, selectively omitted to favor the abuser, or false information is presented with the intent of making victims doubt their own memory, perception and sanity.

"fishing" (or "fishing expedition")
(idiomatic, law, US, informal, pejorative) A non-specific search for information, especially incriminating information.

Now that I'm thinking about it, "subterfuge" might be the best option:

"sub·ter·fuge ˈsəbtərˌfyo͞oj noun deceit used in order to achieve one's goal.