I have heard many stories from many independent sources about Rabbis receiving teachings via individual revelation in the form of Gilu'i Eliyahu (i.e. "the appearance of Eliyahu") or Gil'ui Maggid (e.g. the AR"I, the Ba`al Shem Tov, R' Shalom Shar`abi, R' Yosef Qaro, etc.).
Does Halakhah require one to believe that such forms of individual revelation are indeed possible? If so, how can one verify the authenticity of such claims?