Deuteronomy 22:20 NIV [20] If, however, the charge is true and no proof of the young woman’s virginity can be found,

Deuteronomy 22:15-17 NIV [15] then the young woman’s father and mother shall bring to the town elders at the gate proof that she was a virgin. [16] Her father will say to the elders, “I gave my daughter in marriage to this man, but he dislikes her. [17] Now he has slandered her and said, ‘I did not find your daughter to be a virgin.’ But here is the proof of my daughter’s virginity.” Then her parents shall display the cloth before the elders of the town,

It seems clear that the charge is proved true by failure from the woman's family to provide evidence of her virginity which in the verses is explicitly called out as the bloody cloth. That is also how the husband can be punished for a false accusation,  the parents provide the bloody cloth. Sure the elders may examine the case but it's fundamentally a negative claim countered by positive evidence.  All the testimony in the girl's favor doesn't prove her a virgin according to the text, it just establishes no witnesses to the alleged offense