A hand on verifier

A verifier was a co-mingling of computer and sensor technology used to verify the accuracy of statements spoken by the individual being verified through psychological changes.

When lieutenant commander Montgomery Scott was being tried for the murder of Kara, it was pointed out in court that "the verifier showed he was truthful when he said he didn't kill her." In fact, after a second series of questions that Scott was unable to recall the answers to, a request was made of the computer for an accuracy scan, which verified that Scott was "relaying accurate account. No physiological changes." (TOS: "Wolf in the Fold")

When Thelev was discovered to be an Orion spy serving in the Andorian delegation aboard the USS Enterprise in 2268, he was subjected to questioning under a verifier scan and given a truth drug. Unfortunately these tactics did not reveal anything, which suggested to Spock that "his mind has been so conditioned as part of a preconceived plan." (TOS: "Journey to Babel")