This might be one of the easiest supernatural claims to demonstrate (if it were possible), because pretty much all you need to do is confirm that (1) someone is in the air, (2) they aren't attached to anything, and (3) they don't have any physical device that could explain levitation. #3 would actually be a pretty noteworthy claim all by itself.
Scientific documentation
If levitation can be reliably reproduced, it should be relatively trivial to scientifically document this, to a point that belief is justified for the general public (which hasn't happened).
Just off the top of my head, reliable documentation of this would entail:
A reasonable sample size
A controlled setting (e.g. make sure there aren't any wires hanging from anywhere)
Set up, observed and documented by reputable and unbiased* scientists
* As unbiased as possible. In this case, one might look for scientists without an existing belief that levitation is a thing, or those who demonstrate skepticism and critical evaluation with respect to the claim (which should also be reflected in their reports of the events).
Reliable documentation in various forms (video evidence, written reports, other measurements, etc.)
Possibly a scientific control in the form of e.g. well-trained illusionists intentionally faking levitation to try to trick the observers (which the observers should then identify as fake)
Results that can and have been reproduced by others
Personal justified belief
A simple observation of levitation could conceivably justify belief in levitation, but that explanation would be competing with trickery, illusion, hallucination and false memory. To make levitation more likely than any of those, one could consider the elements mentioned above:
- Did you see this happen multiple independent times?
- Are you in a setting where they could've conceivably been connected to a wire or something?
- Do you know how these things could be faked and what you should look out for to identify that?
- Do you understand enough about illusion, hallucination and false memory to be confident in saying it's none of that?
- Did other people also see it, and how closely do they corroborate your account? This also relates to false memories, because one person's account could subconsciously influence someone else's account.
Justified belief without a formal scientific process
One may also be able to justify belief by simply having levitation that's independently filmed by many unrelated and reputable people, and which can't reasonably be explained by trickery or illusion.
But if you don't have a controlled setting, set up and observed by people who know what they're doing, it's harder to eliminate those alternative explanations.
What is the actual explanation for this?
Let's say you've got yourself a justified belief in levitation. (Crucially, we haven't gotten to this point yet.)
To attribute this to demons (possession) or psychic powers or whatever else would require additional evidence. If someone is able to levitate at will (and this is confirmed as actual levitation), some form of psychic powers might be the most reasonable explanation. A claim of demons would be positing an entirely difficult realm, which would come with a whole lot of implications for how this would affect observed reality, and may come with other things we'd expect to see elsewhere - it's conceivable that one could make a case for that using other evidence, but it certainly doesn't follow too closely from just levitation.