Oftentimes a witness testifying has either been convicted of a crime or they are charged with a crime. If the witness has a conviction, then look to SCRE 609 about whether or not that witness may be impeached about the conviction.
But what if the charge is still pending? Well that wouldn't qualify under 609, but it could qualify under SCRE 608 - specifically the witness could be biased if it is a criminal case and they are testifying on behalf of the State (i.e., the witness might testify favorably for the State in hopes of getting a lighter sentence - or that's the theory).
A recent S.C. Court of Appeals case explained that the defense has a right to cross examine the witness not just about the charge, but also about the potential sentence they are looking at: