How does Sam Know?

[Inziladun]

Originally Posted by dasbill

When Faramir tells Sam and Frodo that Boromir is dead they are stunned because they were not there when Boromir took three arrows in the chest. Yet a few scenes later when Sam is pleading for Farmir to let him and Frodo go he blurts out the desire to take the ring killed him. How since neither Faramir nor Sam or Frodo knew the timing nor matter of Boromirs death did Sam now that Boromirs momnet of weakness led to his killing by orcs?

Are you referring to this quote?

Now I watched Boromir and listened to him, from Rivendell all down the road--looking after my master, as you'll understand, and not meaning any harm to Boromir--and it's my opinion that in Lórien he first saw clearly what I guessed sooner: what he wanted. From the moment he first saw it he wanted the Enemy's Ring!

TT The Window on the West

Later we have this also from Frodo:

Would you have me come to Gondor with this Thing, the Thing that drove your brother mad with desire?

TT The Forbidden Pool

In the first quote, Sam tells of his growing suspicion of Boromir's intentions leading up to the Breaking of the Fellowship, already knowing full well what he had done to Frodo before his death. That is before Faramir learned the truth about the errand of the Fellowship.
The second quote was spoken after, when Faramir, Frodo and Sam had discussed the event. I don't see the issue you speak of.
By the way, welcome to the Downs!