Truth is one of the three interrogation options given during suspect questionings. It is used to agree with the suspect's response and when the player does not suspect they are withholding information.
Truth is used by Phelps to agree with a suspect's story or response to a previously asked question. Selecting "Truth" will not accuse them of lying, obviously, and Phelps will pursue the lead which they have given in their response.
Truth should be used when the suspect displays no sign of lying. This can be detected by looking for constant eye contact, a lack of fidgeting, or a straight facial expression.
As explained by Stefan Bekowsky on the first Detective case of the game, choosing "Truth" will offer a much more gentle approach to the suspect in question and is the least aggressive of the three, with Doubt and Lie usually leading to Phelps insulting or berating the suspect.