John Gill Commentary Deuteronomy 19:18

"and the judges shall make diligent inquisition: and, behold, if the witness be a false witness, and have testified falsely against his brother;" — Deuteronomy 19:18 (ASV)

And the judges shall make diligent inquisition Into the case before them, into the nature of the evidence and proof that each witness brings for or against; so the Targum of Jonathan, "the judges shall interrogate the witness, by whom these things are said, well;" shall thoroughly examine the testimony given, and look carefully into it: and, behold, if the witness be a false witness, and hath testified falsely against his brother; it appears plainly by full evidence that he has testified a falsehood of him.