How does Professor McGonagall know about the Golden Trio's conversation?

Question

In The Order of the Phoenix, there's this conversation between Harry and Professor McGonagall at the end of the twelfth chapter.

"Didn't you listen to Dolores Umbridge's speech at the start-of-term feast, Potter?"
"Yeah," said Harry. "Yeah... she said... progress will be prohibited or... well, it meant that... that the Ministry of Magic is trying to interfere at Hogwarts."
Professor McGonagall eyed him for a moment, then sniffed, walked around her desk, and held open the door for him.
"Well, I'm glad you listen to Hermione Granger at any rate."

Did some of you notice that the professor is talking about Hermione, when she can't possibly know the talk between Harry, Ron, and Hermione at the feast? When they were talking about it, they were sitting at the Gryffindor table, weren't they? Professor McGonagall was sitting at the professor's table, which is far away from them and it's really impossible to hear student's conversations. Well, I wonder how she knows. The professors really couldn't be spying on students, could they?

Answer 1

She doesn't know that it happened, but she can make an educated guess

The implication is that McGonagall (correctly) assumed that Harry hadn't actually understood the subtext of Umbridge's speech, and is regurgitating an analysis given to him by somebody else. Since Hermione is one of his best friends, as well as the sort of person who would have understood that subtext, it's a pretty safe bet that she's the one who told him; no espionage necessary

There are a few clues to this effect:

• Harry is quoting Hermione more-or-less verbatim; McGonagall is likely to recognize the words of one her long-time students, even though Harry's the one saying them¹

• Harry's incredibly naive approach to dealing with Umbridge makes it clear that he either doesn't understand the gravity of her appointment (that is, the implications of the Ministry foisting a teacher upon Dumbledore, who otherwise acts largely autonomously to the government), or he just doesn't care. Either is as likely as the other, knowing Harry, which brings me to:

  • As a subpoint to this, it's worth pointing out that McGonagall subltly tests Harry's perceptions earlier in their conversation; by his reaction, Harry clearly doesn't really understand the danger posed by Umbridge:

    "Misbehavior in Dolores Umbridge's class could cost you much more than House points and a detention."

    "What do you - ?"

    "Potter, use your common sense," snapped Professor McGonagall, with an abrupt return to her usual manner. "You know where she comes from, you must know to whom she is reporting."

    ^{Harry Potter and the Order of the Phoenix Chapter 12: "Professor Umbridge"}

• He struggles to remember what Umbridge actually said (note the ellipses in his speech, indicating pauses for recollection and uncertainty), despite having heard it only the night before, but easily recalls the one-sentence summary. Clearly the speech didn't make much of an impression on him.

  • Recall also my sub-point above: there's an incongruity between Harry's responses in the two conversations. At first he has absolutely no idea what Umbridge's presence represents, but when prompted with a specific memory he suddenly does. Again, sure indication that he didn't puzzle it out on his own

Bear in mind that McGonagall has been teaching Harry and Hermione for four years by this point; she sees how they contribute in lessons, and how they perform on homework assignments and exams. It seems quite reasonable that she'd be able to guess this.

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¹ Credit where it's due; Valorum made this same point in a now-deleted answer