Page 763, a conversation between Mario Incandenza and his mother Avril[1].

"Moms?"
"I am right here with my attention completely focused on you."
"How can you tell if somebody's sad?"
A quick smile. "You mean whether someone's sad."
A smile back, but still earnest: "That improves it a lot. Whether someone's sad, how can you tell so you're sure?"

Let me answer you, Mario. When an author writes a 981-page novel with a hundred-plus pages of endnotes that's about depression in the way that "Right Ho, Jeeves" is about Jeeves and Wooster, that's how you know somebody's sad.

When a reader plows through that same work in three weeks, obsessing over details, googling a series of words that turn out to be invented[2], using a second bookmark to track the endnotes[3], and then reaches the end and just kinda stares at the book and flips idly to passages they particularly liked, that's also a good sign that somebody's sad.

I spent three weeks inhaling this thing, laughing at its jokes, struggling with its syntax and language, but most importantly feeling all the emotions suffered by its exquisitely-drawn characters. I'm not even sure I could recommend it to other people without a detailed interrogation of their likes and dislikes and a good feeling for their literary pain threshold. And it was very difficult reading. But Goodreads' 5-star rating is "it was amazing" and yes, yes it was.

[1] Avril co-founded the Militant Grammarians of Massachusetts, which both explains her reflexive correction of her son's grammar and speaks to the glorious absurdist depths of the worldbuilding.
[2] Like "fantods", a word invented by Wallaces's mother, who was a prescriptive grammarian but not, to our current knowledge, militant.
[3] Something all readers agree is essential, and which almost dictated the choice of physical paperback over ebook.