How does the impeachement of witness work when the witness has had prior charges and remaing charges? And how do you check a witness credibility?

It depends on what the charges were.  If he has been convicted of a felony or a crime of moral turpitude (that is a crime involving lying, cheating, or stealing) then that will put his honesty into question and you bring that up on cross-examination.  It does not mean he is lying, but it is something that the judge or jury can consider in weighing his credibility.  However, if his charges are still pending then you cannot use them against him, as he is innocent until proven guilty.  Of course, if you have been charged with a crime you definitely should have a lawyer and your lawyer should know all of this.