It's commonly said, mainly here on Writing SE and mainly by Mark Baker's answers, that "a story is a promise". The beginning of the story sets the promise and the ending fulfills it. A story that doesn't fulfills its promise is unsatisfying.
I understand this when I intentionally make a "promise" in the beginning. However, how can I know whether I made an unintentional promise, i.e. a promise that I made without even noticing it?
Pay attention to your editor's/beta readers' reactions. Ask specifically:
If you get good answers across the board, you're probably fine.
What you don't want is for people to say "Yeah, I really thought Greg and James were going to get together, because Greg was always jealous of James's wife and people kept making jokes about Greg and James dating, but nothing came of it." Or "you spent all this time talking about how terrifying the wargs were, but then Anne cast the Warg Repellent spell and they were all dead in two paragraphs."
Those are unintentional promises — expectations you set up but didn't follow through with, potentials which don't pay off. They can be major or minor, but asking those questions can teach you what to look for.
ETA To address Andrey's excellent comment: A subverted expectation is deliberate: you think the good guy is going to put down his weapon, but instead he shoots the hostage. Your expectation is that he was a pure good guy who puts the hostage's safety first, but instead he turned out to be (or became) a more gray guy who is putting the larger good or the mission first. If you re-read, you can trace the development of his changing morality or you can see where he was never all that good in the first place.
An unfulfilled promise is when you spend three seasons setting up a romance between Sherlock and John, and then in season 4 it becomes The Mary Morstan Bro No Homo Show. It might be queerbaiting or it might be bad writing, but it's clearly not where the story was going. There was no narrative hint beforehand that the romance was going to be abandoned, and the focus on a third character doesn't follow from any previous character or plot threads.
What a fascinating question. I suspect that the answer is that you can't with perfect certainty. There will probably always be readers who will pick up your book hoping for one thing, thinking in the early going that they are going to get it, and then being disappointed in the end when they don't. In fact, I think we can see this happening quite often in user reviews of books and movies on many different sites. A lot of the low rating you see are attached to comments that say, in one way or another, "not what I thought it was going to be".
I'm not sure there is any way to avoid this entirely. A story is an experience and there are lots of experiences in life that we think are going to be fun at the beginning but then find we don't enjoy them as the experience progresses. For some people the roller coaster is great fun until halfway down the first hill.
But I think genre plays a huge role here. In fact, I'm pretty sure that I have talked about a genre being a promise just as much if not more than I have talked about stories being a promise. Each genre and sub-genre is defined by a set of conventions and those conventions promise a certain kind of experience. I think there is a tendency in beginning writers to think they can win an audience by defying the conventions of a genre, but in fact there are very few successful works that do that, and I would argue that most of the ones that are said to do so, don't really do so as much as people crack on that they do.
And this, of course, brings us back round to the perennial discussion about the role of originality in building a writing career. There is generally far more money in making a better burger than starting a highly original place selling monkey brains and fried scorpions. Fulfill the promise of the genre with a really well executed story and you won't disappoint your readers.
EDIT: Reading Lauren's answer puts a further thought in my mind. Sometimes the promise that the reader sees comes from the reader or from the culture. Sometimes a particular kind of story becomes popular and then every story that comes along that has elements of that popular story, people see the promise of that popular story in it. When that promise is not fulfilled, those same people can be quite vicious in their condemnation of the story for not being the story they wanted it to be.
Actually, this business of taking a story for the kind of story that the culture is expecting at a particular moment may be quite common. So many people took LOTR as being about the bomb because of the time when it was published. It is, in fact, a deeply Catholic examination of the nature of temptation and the role of love, (themes which clearly occupied much of the thought of Tolkien's circle, particularly Lewis as evidenced by The Screwtape Letters). But little of the book's reception and popularity seems to have had much to do with this. Instead many different groups seem to have found their own promises fulfilled it it.
There are other books, of course, were you can very clearly see that they fulfilled the expectations of the moment in which they were written.
Characteristics you introduce are often unintentional promises.
To be too obvious here, if I write a character from the beginning that was a long distance sniper in the Marines, but he is leaving the military to become a safe boring accountant, readers are going to expect him to shoot somebody. Otherwise, why did I make him a sniper? Perhaps I did that for the contrast and to give him something to "escape", PTSD from being under fire while shooting dozens of enemy soldiers.
But that characteristic is a promise: A sniper should snipe, a Marine should fight, Don Juan should seduce multiple women, and a woman spy trained to trade sex for information should be shown doing that in the story.
This is similar to "show don't tell", but basically in the opposite order: If you tell us something unusual about a character or something that has dramatic potential, the reader expect that to pay off sooner or later. If you tell me Bill is forgetful, he better forget something. If you tell me nothing about Chuck's height or build, I expect him to be average, and don't expect him to turn out to be six foot eight. If you tell me he is six foot eight, I expect that to figure into something at some point.
What you write is there for a reason, and this is unlike real life in that sense. IRL you can be friends with some giant person and that never has any dramatic consequence. In a story, such an unusual characteristic makes the reader expect some consequence.
Be cautious with the little details, the set dressing. You can describe a normal scene for pages upon pages, and all you will do to the reader is draw a picture in their minds.
But the moment you describe something out of place in that scene, the moment something jarring comes up, that's the moment you make a promise about it.
You can describe the furnishings of a room at great length, the decorations on the walls, the hunting trophies... but the moment you describe a gun on the wall, you are describing an implement that will be used in your story. Yes, even if you're writing about the American Wild West.
In general, it's obvious from context. In a thriller set in the US about terrorism, some swarthy men with grizzled features wearing cloth head-coverings will clearly be a plot point; in a romance set in the middle east they may just be local color.
But it can cause trouble when, to you, something is so entirely normal, you don't realize it's out of place in that particular setting. Someone steps out of their shoes as they enter a building (are they being sneaky or simply polite?); someone is driving 5mph over the limit (are they dawdling, or speeding?); there's a dead animal lying by the street (a commonplace or a tragedy?); and so on.
"Come on in, I'll put the kettle on." To an English person, this just means a friendly welcome, but to others might suggest that boiling the kettle may be the point of the visit.
