Originally Posted by
Jessiwuzzle Lives
I saw it a second time yesterday. Dumbledore does say the diary is a horcrux and so is the ring and the locket. They had to figure out what the other horcruxes were in the book too. Dumbledore didn't give any clues other than that they would be special or important things.
in the book he tells harry what (he at least suspects) they are. that's how we know one of them is nagini, the snake. one is the cup, the locket, and something belonging to hufflepuff / ravenclaw (can't remember which) & the others we already know about.