What clues do Aragon, Gimli and Legolas use to help determine the fate of the hobbits? What do they learn from these clues?

From the text:

He surveyed the green lawn, quickly but thoroughly, stooping often to the earth. 'The Orcs have been on this ground,' he said. 'Otherwise nothing can be made out for certain. All our footprints are here, crossing and re-crossing. I cannot tell whether any of the hobbits have come back since the search for Frodo began.' He returned to the bank, close to where the rill from the spring trickled out into the River. 'There are some clear prints here,' he said. 'A hobbit waded out into the water and back; but I cannot say how long ago.'

'How then do you read this riddle?' asked Gimli.

Aragorn did not answer at once, but went back to the camping-place and looked at the baggage. 'Two packs are missing.' he said, 'and one is certainly Sam's: it was rather large and heavy. This then is the answer: Frodo has gone by boat, and his servant has gone with him. Frodo must have returned while we were all away. I met Sam going up the hill and told him to follow me; but plainly he did not do so. He guessed his master s mind and came back here before Frodo had gone. He did not find it easy to leave Sam behind!'

'But why should he leave us behind, and without a word?' said Gimli. 'That was a strange deed!'

'And a brave deed,' said Aragorn. 'Sam was right, I think. Frodo did not wish to lead any friend to death with him in Mordor. But he knew that he must go himself. Something happened after he left us that overcame his fear and doubt.'

From these clues they learn that Frodo and Sam have left together by one of the boats on the river.