וְאִם־אֱמֶ֣ת הָיָ֔ה הַדָּבָ֖ר הַזֶּ֑ה לֹא־נִמְצְא֥וּ בְתוּלִ֖ים לנער [לַֽנַּעֲרָֽה׃]
그 일이 참되어 그 처녀에게 처녀인 표적이 없거든
Rashi on Deuteronomy
ואם אמת היה הדבר BUT IF THE THING BE TRUE — proven by evidence of witnesses and after legal warning that she had committed adultery after her betrothal (Ketubot 44b).
Siftei Chakhamim
Through witnesses, and with a pre-warning, etc. Otherwise, why is she stoned? For even if it is true that she behaved promiscuously, perhaps she did so before her marriage. And even if she behaved promiscuously after her marriage, perhaps she acted without witnesses or pre-warning. Therefore Rashi explains, “Through witnesses, etc.”
Rav Hirsch on Torah
V. 20. ואם אמת היה הדבר הזה וגו׳, es hat der Vater nicht die Zeugen des Mannes vermittelst Gegenzeugen durch הומה zu entkräften gewusst, oder es sind die Gegenzeugen des Vaters selbst wieder vermittelst anderer Gegenzeugen als עדים זוממין erwiesen worden.