In Season 2 of 13 Reasons Why, Courtney Crimsen is asked during her testimony:
Courtney, some people might wonder, if you didn't tell the truth for so long, how do we know you're telling the truth now?
I could be misunderstanding the principles, but it seems like the principle of criterion of embarrassment or the principle of declaration against interest (Oh, wait, should this perhaps be Party admission? Statement against interest?) answers the question: Courtney Crimsen is incriminating herself, ruining her own reputation, exposing an embarrassing truth about herself, etc, and that is why her testimony is very likely true.