When God addressed Moshe there was an accompanying miracle (and there's midrash about Avraham's case being special too), so it was pretty clear what was happening, but how did the other prophets come to know that it was God who was speaking to them -- not a delusion, not a trick, but divine communication? Was it immediate, or could there have been some period during which the prophet wasn't yet sure what was happening to him?
(This question arose out of why did Yonah run away?.)