In a trial for perjury, the person would be asked, under oath, how they came to believe the information they claim they believed when they gave the testimony th

In a trial for perjury, the person would be asked, under oath, how they came to believe the information they claim they believed when they gave the testimony that is under dispute. If they actually just made it up or guessed, they would either have to say so, or commit perjury by lying about it. And if the lie was, for example, that so and so gave them a briefing and they understood them to be saying X, then so and so would be called as a witness and their testimony about whether such a briefing took place, and what was said if it did, would be given in evidence.