How do we determine and designate a person as a fasiq (transgressor) to verify their testimony, fearing that some may consider him truthful while others deem him a fasiq? So, how do we distinguish?

The transgressor whose report must be verified is one whose transgression is due to lying. If a transgressor’s lies outweigh his truthfulness, then his report and testimony are not accepted. As for the transgressor whose transgression stems from other aspects, yet he strives for truthfulness, his report and testimony are accepted; otherwise, rights would be obstructed, and truthful reports invalidated. If a transgressor's lies are rare, then there are two opinions among scholars regarding the acceptance of his report and testimony.