Agent of S.H.I.E.L.D.
Dec 7 2019, 7:48pm
Sauron's Ring was not exactly common knowledge. As Gandalf said, what he told Frodo was for the most part known only to the Wise; and apparently nobody had ever read the Scroll of Isildur until Gandalf found it.
While Faramir was studious (as was his father), it's not likely that they knew any particulars, other than the rumor (3000 years old!) that Isildur had recovered some talisman of power from Sauron's body. And it's not likely Isildur told many people; according to The Disaster of the Gladden Fields, he only told his eldest son Elendur about it, and Elendur didn't survive.
Note also that nobody in Gondor, neither Denethor nor either son, had any clue what the "Isildur's Bane" of the dream was. While Boromir wasn't the bookish type, if Denethor had figured it out he certainly would have told him before sending him north! (In fact, the book heavily implies that Denethor had never heard of the Ring until Gandalf told him of Frodo's mission on the eve of the Siege; and Denethor's scolding of Faramir happened after that)
Faramir was shrewd, and he doubtless guessed near to the truth; but I don't think all the tumblers clicked into place for him until Sam's outburst.
(This post was edited by Solicitr on Dec 7 2019, 7:51pm)