In Malachi 3:23 -24 it says
23. Lo, I will send you Elijah the prophet before the coming of the great and awesome day of the Lord,
24. that he may turn the heart of the fathers back through the children, and the heart of the children back through their fathers-lest I come and smite the earth with utter destruction.
How will one know that it is Elijah? It says "I will send" , where is he being sent from, where will he appear, will he be known to all or just those that G-d has chosen?