After Sauron finished crafting the One Ring and placed it upon his finger the elves knew of his treachery, and his intentions. But how did the elves know where Sauron crafted the One Ring? And how did they know that throwing the One Ring into the fires of Mount Doom would destroy the One Ring? How did they know that there was no other way in existence to destroy the ring?
No one ever tried to destroy the One Ring before. Isildur took it shortly after cutting it free from Sauron's hand. The ring corrupted him and Isildur would never allow it to be handled by anyone else. From there he lost the ring when it betrayed him in the river. After that it came into the possession of Gollum / Sméagol. We know Gollum never tried to destroy it. Then Bilbo found it. Other than when Gandalf threw the ring into the fireplace no one ever tried to harm the ring.
And this leads me back to the beginning. How did the Elves know where the One Ring was forged, and how did they know that the fires of Mount Doom were the only way to destroy the One Ring?