Prior the the Men of the Great Assembly, could halacha be determined by prophecy?
Answer:
The Torah was given over though a prophecy to Moshe Rabbeinu and all of the Jewish people. Many halachos were also taught to us by the Neviim, who received them through prophecy. At this point, before the period of the Tanaim there were no disputes or unclarity in Halacha. After the period of Nevuah, although there was still a lower form of a Divine message , such as Ruach Hakodesh or Bas Kol, the halacha was not determined by them.
Sources:
see here for an extensive discussion on this topic:
https://dinonline.org/2015/02/27/not-in-heaven-supernatural-revelation-in-halachic-rulings/
