Xjyuk

I feel like the title is worded really bad, but I seem to be unable to find a better way. I am not a native speaker.

Scenario A: A character is in an exceptional state of mind (i.e. drug use etc.) and envisions vivid imagery, sounds, smell etc.

Scenario B: The same character later encounters typical fantasy events - let's say a monster.

How would you advise to differentiate between the two, if you cannot show the actual drug use? In my example the drug is administered while the character is unaware of it.

In a mundane story we could let the reader decide which events are plausible and which are not. But in a fantasy story, where it is not exactly known to the reader, what is possible and what is not?

Think about how dream-like or drugged states are portrayed--hazy, halting, illogical. Nothing in an imagined state is solid; time skips around, scenery/environment changes very quickly, people and faces morph into other people and things.

Now compare this to the reality of the monster your character encounters. They may have a moment of, "Wait, is this another hallucination?" before coming to a realization that, no, this is very much a real thing that is happening and they need to run.

What I'm advising is, instead of letting the reader decide what's real or outright telling them what's real, have the character struggle to decide what's real and, through observation, come to a conclusion. Use the character's past experiences with the sensation of an out-of-body experience or hallucination to aid the character's realization, and then use that knowledge in the rest of the story's context--perhaps the character can now realize when they're inside a hallucination, or they can now lucid dream.

Have fun with your story, and remember that not everything has to be explained to a tee. This isn't CinemaSins, we aren't going to ding you for not throwing the information in the faces of the readers/viewers!

If the main character isn't sure whether what they're seeing is a drug-induced fantasy or a real occurrence, perhaps the reader doesn't need to know either, at least not at first.

If a character has been drugged and is hallucinating, what's happening is "real" to him. He experiences things and responds to them. But there might well be hints that something isn't right: the character's body not responding as he expects, jumps in what is going on without a hint of what happened between, a lack of logical sequence of events. The character might even notice those things and wonder at them. Eventually, those "wrong" elements would build up to an understanding that this isn't really happening (or else, your fantasy world is really weird). An example of things not following the kind of sequence they would in real life:

Then he heard a noise in the distance. At first he thought it was a great wind coming over the leaves of the forest. Then he knew that it was not leaves, but the sound of the Sea far-off; a sound he had never heard in waking life, though it had often troubled his dreams. Suddenly he found he was out in the open. There were no trees after all. He was on a dark heath, and there was a strange salt smell in the air. Looking up he saw before him a tall white tower, standing alone on a high ridge. A great desire came over him to climb the tower and see the Sea. He started to struggle up the ridge towards the tower: but suddenly a light came in the sky, and there was a noise of thunder.
_{J.R.R. Tolkien, The Lord of the Rings, book I, chapter 5 - "A Conspiracy Unmasked"}

The character is "suddenly" out in the open. There is lightning and thunder, but no hint of coming storm or anything like that has been given before. This is "dream logic" as @DanBron puts it in a comment.

If a character experiences something that is extremely unusual for them, they might discount it as their mind playing tricks, especially if they have experienced being drugged or having vivid dreams before. Consider: if you saw a dragon over your city, would you immediately believe your eyes, or would you think you're seeing things? When the dragon affects more senses than just sight - when you hear its roar, when you feel the heat of its fire and the stench of its body, most crucially - when you hear people screaming, that is other people responding to its presence - then it becomes easier to believe that what you're seeing is indeed a dragon. (Even so, The Chronicles of Thomas Covenant the Unbeliever is built around the main character refusing to believe that the fantasy epic he's experiencing is not in fact a hallucination.)

If, nonetheless, you wish to maintain the distinction right from the start, you can start your dream sequence with "it seemed to him that" or a similar phrase. That would set up the whole ensuing sequence as not-real. An example:

In the dead night, Frodo lay in a dream without light. Then he saw the young moon rising; under its thin light there loomed before him a black wall of rock, pierced by a dark arch like a great gate. It seemed to Frodo that he was lifted up, and passing over he saw that the rock-wall was a circle of hills, and that within it was a plain, and in the midst of the plain stood a pinnacle of stone, like a vast tower but not made by hands. On its top stood the figure of a man.
_{J.R.R. Tolkien, The Lord of the Rings, book I, chapter 7 - "In the House of Tom Bombadil"}

In this case, we are explicitly told that what Frodo sees is a dream.

Note also that in fantasy literature, a dream might contain a vision of things to come, or of things that happened before, or are happening elsewhere. (As in the LotR example above.) Thus the line between dream and reality blurs even more.