Within the novel, Faramir tells Frodo and Sam that Boromir has died, and they fill in the gaps around what happened prior to that.
Faramir smiled grimly. ‘Then you would grieve to learn that Boromir is dead?’
They work out that the days match up so that Faramir's vision is from the same day as they left.
‘But the day when you heard it blowing, if your ...