There are several ways to address this, so I will try to list/explain all the ways I'm familiar with.
Just Have The Narrator Say So
This only really works if there is a narrator, and the reader has sufficient reason to believe they are reliable. But it is the simplest.
Provide an External Source of Truth
This has the flaw of not being inline (presented as soon as it's relevant). However if a book comes with a Map, or Codex (Encyclopedia); The reader will have no reason to doubt it as a source of absolute truth. Some readers might miss it, but it will be there for anyone to easily tell/prove that something is a fact.
Create an Aside
In a play, this normally takes the form of all the actors freezing while one of the characters, or the narrator, steps out to speak directly to the audience. In Literature, this is fairly rare, but there are two examples I can give.
- A text box on the side of a page or between paragraphs.
- A block of text formatted differently, and separated from the main text with plenty of white space.
The main idea remains the same. This text is visually very distinct from the main text, and often uses a very blunt-matter-of-fact tone; Usually also a different font. The reader should be able to tell this text is a type of Aside before they even read a single letter of it.
This is very format (display) dependent, so requires you to actually have control over the display of the text. Also requires to be used every so often so that it isn't jarring/out-of-place.
Very Heavily Imply It
While strictly not an "Absolute" truth, I feel it is worth mentioning because in real life, there is no such thing as absolute truths. We can't prove that ghosts don't exist, but the lack of randomly floating objects or photos makes a pretty compelling case. We also have no idea how or why gravity works, but it hasn't failed us yet in all of recorded history so why would it stop tomorrow? Rocket science is based on that trust! However, for all we know, gravity could have an expiration date.
Creating a long and consistent case that something is a fact will be good enough for most readers; and for those who aren't convinced? Do you really want to tell them "Santa isn't real"? Let them have their fun theory.
The down side of this is that it somewhat requires the entire story to act as the "source of truth", so doesn't really help in a short term to reassure the reader of a fact as they are reading.
Foreshadowing / Good Chapter Titles
This one is a little close to implied, but even if you have a first person narrative, the author still has ways to talk to the audience.
Even in a first person narrative, Chapter Titles are from the author, not the narrator. So even if the narrator is unreliable, chapter titles can provide strong clues or facts about the current situation. (In some cases, like the the game Dead Space, the First letter of each chapter is used to spell out a secret truth)
Foreshadowing that something is the case, if you are able to do so, can help reassure the reader that something is real. For example, in the story for Scrooge, the main character is told he will be visited by 3 ghosts. When exactly 3 ghosts appear to him, this tells him and the reader that this wasn't a fluke, and that everything was real/intended.
This is good for 1-off truths but can be a little too subtle for some readers. It also is limited for very short/simple facts. It also requires some creative thinking, so may be hard to implement in practice.