How did they know that facing the Kaʿbah was the truth?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. The command to face it is found in their scriptures. [Abū al-ʿĀliyah]
2. They know that the Sacred Mosque is the qiblah of Abraham. [Ibn ʿAbbās]
3. It is found in their scriptures that Muḥammad is a truthful messenger from Allah, so the commands he gives must be true.

1. They know that some revealed rulings may be replaced by others (naskh).