Ever since the Gladden Fields, Sauron and Saruman have been looking up and down
the Anduin. Frodo was lucky Gandalf 'shoved' him out of the Ring sight
otherwise Sauron would have sent his legions to Amon Hen.
Sauron's search is like a wave-sends shudders all over the land. Frodo was
aided by the Seat of Seeing and Gandalf's long distance telepathy, but
ultimately took the Ring off.
Gandalf deduced that since Aragorn and the Hunters are in Fangorn, and Merry
and Pippin are with Fangorn that Frodo must be alone and Sam sent home. Yet
Aragorn's answer makes Gandalf smile- Sam is still there to watch over Frodo.
Gandalf knows something of each of the Three Hunters- Aragorn, Gimli and
Legolas would have gone to Mordor with Frodo if Merry and Pip were safe.
Let's just say it's Gandalf 'talking to the wisest person' in the room
again-himself.
Gandalf has been alive in Middle Earth longer than everyone else in the
Fellowship,even Legolas- he's seen these things before. His guesses are not
wild- he is Sauron's opponent in this chess game and sees all the pieces.
Gwaihir might have seen the Fellowship being attacked, but Aragorn minus Frodo
means that Frodo escaped both Saruman and the Fellowship. FOr someone like
Gandalf, it's easy to guess.
Sauron's Eye searches the Anduin with only Lothlorien being clouded to him-
Frodo is sensed by Sauron, but not long enough to be 'contacted'. Sauron sends
Grishnakh and his boys to meet up with the kidnappers, who have a halfling
pair-he wouldn't send anyone if he didn't know of the Ring being used somewhere
around there. THe Nazgul are also spying around these parts- they would have
seen the
Uruk-hai.
There is but one task remaining in Arda-we,the Dragons of Valinor will make it happen.
Lone resident of the Republic of Gondor
PO-TA-TOE-S not PO -TAH-TOE-S!