If there is a magician on the stage, you can safely presume there is no actual magic involved, even though you do not know how exactly the trick works.
For actual flux pinning, the first thing you would do is show what happens if you put the thing upside down. It should stick. Even if it does not, you would show that it does not.
The point of the illusions in a good magicians act is that they look like magic. If the illusions look like invisible strings/etc, then they were poorly done. Even if you know there must be a string, it shouldn't look that way.
So what I'm saying is that even if it's reasonable to deduce that this supposed magnet is being suspended from a string, it doesn't look like it is. If it is fake, it's a well-done illusion not a shoddy illusion.
Whether it "looks like it's on a string" is highly subjective. Suppose I primed you by saying: Look at this speck of dirt on a string, does it look like a speck of dirt on a string to you?
For actual flux pinning, the first thing you would do is show what happens if you put the thing upside down. It should stick. Even if it does not, you would show that it does not.