What does Denethor already know at Council of Elrond

[Great Auk of Numenor]

Still catching up. I am not half-way through the Council of Elrond, at Episode 147.
Regarding Gandalf's attitude about Denethor, is it possible that Denethor treated Gandalf less amiably than either he or his predecessors in the Stewardship had before because he already knew (via the Palantir) that Gandalf was deceiving him? When he arrives in Minas Tirith to look for the Scroll of Isildur, he has already taken up with Aragorn. Does Denethor already know that Gandalf is plotting to supplant him? Further does Denethor sit in his chamber in the White Tower of Gondor and through the Palantir read the scroll over Gandalf's shoulder? I have always been very sympathetic to Denethor, who has indeed spent his sad life in defense of Gondor and Free Peoples of Middle Earth, but is largely ignored by the Elves and the Wise, until they need him to lead his people in a seemingly hopeless war which they exclude him from their councils in pursuing. All the while, they are plotting to replace him. How much does he know of this through the Palantir? He could be watching the Council of Elrond as it happens; not unlikely that he would be following the quest of his most beloved son Boromir and curious to know what the Council said about the dream, etc.

 

[Jim Deutch]

I think spying on Rivendell with the Palantir is far beyond Denethor's ability. Galadriel tells us that Sauron tries and tries, but can't spy on Lothlorien; I expect it's similar for Elrond. Since Denethor is not the hereditary "owner" of the Palantir, his command over it would be lesser. And he purports to not even know the location, except vaguely, of Rivendell before Boromir set out.

Maybe he could read Isildur's scroll over Gandalf's shoulder, though: that would explain his knowledge of ringlore (though it's certainly not the only way he could have gained it). Perhaps Palantir spying explains his knowledge of Aragorn, but again, he has other sources: there must surely be records of Thorondor, who might not have been quite as incognito as he thought.

It's not clear when, exactly, Denethor first took up the Palantir. I like your guess that it was at the time that his elder son set off on a fool's errand far to the north. But I seriously doubt his ability to use it so effectively as to actually spy on the Council.

 

[Odola]

It's not clear when, exactly, Denethor first took up the Palantir. I like your guess that it was at the time that his elder son set off on a fool's errand far to the north. But I seriously doubt his ability to use it so effectively as to actually spy on the Council.

He started to look into it after the death of his wife.

 

[Flammifer]

Denethor implies that he first learned of Aragorn through naive slips of the tongue by Pippin. "And yet in our speech together (with Pippin) I have learned the names and purpose of all thy companions. So! with the left hand thou wouldst use me for a little while as a shield against Mordor, and with the right bring up this Ranger of the North to supplant me."

That would imply that Pippin, rather than the Palantir, was Denethor's source of knowledge about Aragorn.

On the other hand, Appendix A says, "later, when all was made clear, many believed that Denethor, who was subtle in mind and looked further and deeper than other men of his day, had discovered who this stranger Thorongil in truth was, and suspected that he and Mithrandir designed to supplant him."

This would imply that Denethor figured Aragorn out back before 2980 (when Aragorn as Thorongil, left Gondor). This was before Denethor is said (in Appendix A ) to have begun using the Palantir. "After her death (Denethor's wife, Finduilas, in 2988) Denethor became more grim and silent than before, and would sit long alone in his tower deep in thought, forseeing that the assault of Mordor would come in his time. It was afterwards believed that needing knowledge, but being proud, and trusting in his own strength of will, he dared to look in the palantir of the White Tower."

Both accounts imply that Denethor figured out Aragorn without the aid of the palantir. though neither account is certain to be true.

 

[No One in Particular]

Denethor likely figured out who Thorongil was years ago, before using the palantir. From Pippin he learned that the artist formerly known as Thorongil was now traveling south and the idea of claiming the throne of Gondor was definitely high on his to-do list. Pippin himself was unaware of exactly what information he (Pippin) was in possession of, nor was he not aware of what information Denethor was able to glean from him, or that the information would allow him to connect the two men (Thorongil and Strider). Whether the information obtained from the palantir was supplemental to Pippin's account, or vice versa, is speculative, although I tend to think that what Denethor got from Pippin was like the last nail in the coffin more than anything else.