How was anyone who did not receive the nevuah directly, able to authenticate that Hashem indeed gave a Mitzvah – before Sinai? For example -bris milah which became incumbent on Avraham uledoros immediately or Gid Hanashe acc. to R’ Yehudah who holds it began with Yaakov or the 7 mitzvos which were told only to Adam and Noach? There was no national revelation in which nevuah was authenticated to the masses?
True, they didn’t know it from Sinai, they knew it because they trusted Avrohom, and Yackov who told it to them. They knew who they were and their level of integrity, and they trusted them.