It’s extremely difficult to ascertain this information from reading a book because it’ll be blended into the finished story. It’s also a crushing blow to the junior writer when they fall into this invisible pitfall only realized long after the cuts are made. Then they have to scour distant memories for the little bits they removed and the exact wording for each sentence. It’s embarrassing; you feel like you’ve gone geriatric and you mentally abandon what you’re working on to hurriedly plot out a time travel story just to exercise your pain. If you cut once, measure twice.
This problem usually arises when large swaths of a story are cut for pacing and/or lack of purpose. If your story needs to be shorter or there’s a part which stalls the plot dead then by all means cut away. However, there’s that little rebellious voice near the brain stem pleading for you to halt. Oft this is regarded as weakness or a naïve belief in first draft perfection but sometimes it has a point. Not enough to wholly endorse it but enough to reexamine what you seek to remove.
Take an adventure, join about the middle where our party has met, gotten acquainted, developed interpersonal relationships and opinions about one another, and then meet a run of the mill ambush to break up the monotony of their travels. In the course of the attack, the timid, cowardly protagonist fresh from his homestead sees his warrior companion in a rolling tussle with a bandit. The warrior, not trusting the protagonist to act due to previous examples of dangerous pacifism, rolls with the bandit committed to fight on his own. However, the bandit gets the upper hand and is slowly plunging a knife down onto the warrior. Not wanting to see his friend hurt, the protagonist gains the blind recklessness to charge the bandit (perhaps with eyes shut). He shoves the bandit away and in his ensuing struggle the warrior saves the protagonist.
This is stock but could work with the right effort. However, what if nothing else develops from this event? No one else has a character moment, the party isn’t drawn away from the path they once thought safe, none of them die, they aren’t separated, necessary equipment and tools aren’t stolen, they don’t learn anything new about their objective, etc. Sounds like a good thing to cut if the rest of the story can withstand it.
But what about the one good character moment between the protagonist and the warrior? What if the rest of their relationship is kept the same? With the turning point gone there’ll be one interaction where the warrior mocks the protagonist for cowardice, [cut], and then he has a budding respect for the little guy. Why? Not to mention the cowardly protagonist’s sudden character development practicing combat with the warrior against his stated beliefs. Now the rest of those plot threads and everything connected to them fall apart and as the writer you still hold onto the logic from the cut scene to fill the plot hole. That’s enough of a challenge to fix but imagine if you gave the story another two passes and codified it. Ten times worse.
So, take stock of those plot points and add them in around the same part of the story. If you’ve already set the party in an old, spooky forest, have a rotting tree fall upon the unsuspecting warrior only for the protagonist to impulsively tackle him out of the way. He gets to display a little courage and the warrior can mend his pride, and carry along the question, by stating trees don’t fight back. Fits the bill for the plot points, shortens up the story with a quick instance between two characters, and if you had some Tolkien Ents later on it’s practically a gimme.
The inverse of this method is to take the scenes where not much happens and flesh them out with several of the lost possibilities I rattled off in the bandit attack example. While that effort can hone an entertaining and practical scene, it exacerbates the original issue. If that section of the story is too long then it’s a hindrance to add more onto that scene regardless of how expertly weaved. Plenty can be salvaged from a mistake but you have to know when it’s best to drop it and move onto where your efforts will have the greatest rewards.
This thought process can help turn around the forest for the trees writer hesitant to throw their babies out. A lot of effort can be spent on a single piece of worldbuilding but if it’s in a scene where not much else happens then fit it in somewhere else. It’s more effort to weave multiple plot threads on top of one another in the same scene but it delivers a denser punch. If you’re young and still have the energy to be stubborn about those precious story fragments then try to utilize them in a compact and constructive way.
Don’t worry near this much when cutting a line or two because there’s not as much information that can be lost. Maybe you decide to cut a few of the comedic sidekick’s jokes to keep the character from grating on the audience because you’ve learned brevity is the soul of wit and not interrogation room levels of repetition. Some jokes are cut from better cloth so mind which are one-offs and which go full Shakespeare foreshadowing great upheavals or carry on a running joke.
Ultimately, whenever you come across a section of your story to cut just run through the old criteria: Does it advance the plot or reveal character? and highlight any nibbling “maybe” for further consideration for another part of the story. Perhaps they won’t be as critical as the scenario I threw out but it’s a good waste not want not, continuity preserving mechanism to place in your toolbox behind the scissors.