You know the book is good when...
When you've read a lot of books, you kind of reach a stage when you start to notice patterns. You start to see how certain kind of books behave, and in result, it slowly becomes harder for books to fool you. You know what to look for, so even the slightest hints can tick you, and it'll take no time for you to figure out what is going on. You also start to notice patterns that would identify a book as good or bad (most of the time, not completely). So I've collected up a list of my experiences in reading, and how I myself identify the moment to finally realize that I've hit a jackpot.
You get the famous "adrenaline rush"
I genuinely don't know if any other readers feel this (because I've actually never mentioned this to any other reader before), but does anyone else feel that adrenaline rush when reading a good book? This specific kind of adrenaline rush is not achieved through suspense in the book or specific heart-stopping scenes (or cringey ones when the character just does something so stupid), but the kind you get at the beginning of a book, and for me, is the most obvious instinct and feeling I get when I officially start to realize that I'm having a good book in my hands. Is like you get that rush, and that rush signals me to let me know I'm excited, so I know that this is a good book.
Your favorite trope appears
Although tropes are usually regarded as a negative thing, I disagree. Everyone has a list of tropes they execrate (definitely), but at the same time, they should also have a list of tropes they love. Tropes are also useful to help you identify what kind of things you enjoy in a book, so you can more clearly identify what you love and don't love as a reader. For me specifically, if I ever see a (good) hate-love relationship, 9/10 that book is pretty much secured a spot in I-enjoyed-the-book list.
You cannot physically put the book down (literally, no joke)
This is not a literal device, or an exaggeration, and you should indubitably recognize this as a fact if you've ever read a good book. Which is caused by just simply cannot waiting to see what happens. If the book is not that good, you'll notice that you have no problem just reading the book pages a day instead of books in days.
You start having physical reactions to the book
True bookworms always have physical reactions to books. If you laugh at funny scenes, squirm at cringey scenes, squeal and cute scenes more than 20 times at least, then...it's probably a good book.
You finish the book in record time
Similarly caused by not being able to put the book down and because you just NEED to know what happened next...yeah, this is just going to happen (it's inevitable).