On cross-examination there are various ways to impeach a witness. In this day and age there can be quite a bit of information available on line, usually through social media.
But a great potential source is the witnesses record book, which is why it should always be reviewed prior to trial.
Enlistment documents. Did the person give truthful information under oath during enlistment – about criminal charges, or mental health issues, or medical issues.
Drug abuse statements. Did the person give truthful information under oath.
SF 86. Did the person give truthful information under oath, or is there something inconsistent with other record documents. A common one is a listing of prior service drug use which conflicts with the enlistment documents; thus you have a fraudulent enlistment. Now it’s common to say the recruiter told them to lie. Fine, now you have two crimes going to truthfulness, the false statement under oath at enlistment and a conspiracy with the recruiter to commit a fraudulent enlistment.
Overseas screening documents.
Applications for tuition assistance.
Applications for special programs or assignments.
Of course, the TC should be giving you this information if it exists.