The ultimate test is having them draw a picture. If they can visualize imagery then they should be an excellent artist. If they are bad at drawing, then they must have aphantasia.
I mean, I can look at something physically in front of me, like a still life, all I want and still draw it like shit. So I'm not sure what that would prove?
You would be able to reproduce the object very accurately, given enough time. You can simply continuously refine the drawing by comparing the drawing to the object.
If you do not "have it" and you are unable to reproduce the imagery you see in your mind's eye using a pencil on paper, then that is clarifying. Whatever you consider to be visualization is not even close to the same as actually seeing something. If whatever you experience was a "vivid visualization", then you would be able to reproduce it drawing.
I personally don't experience anything that I would consider "visualization", and I am also terrible at drawing, but I can reproduce a photorealistic drawing of a photograph if I am allowed to look at the photograph while drawing.
I think you are confusing two separate issues. I struggle to do still life as well, when the subject is right in front of me. That's not due to eyesight problems. My visualisation is not perfect but is much better than anything I can draw. However, when I have taken art classes and I practised, my skills improved... Much like music or chess. If I stop practising drawing then my skills deteriorate.
I can write about things in far more detail than I can draw them. Both from my imagination and sometimes memory. I can remember the floor plan of certain buildings I haven't set foot in for over thirty years. (Or at least the parts I had access to.) I can remember which direction they are orientated in, and where the sun rose and set in relation to them. As me to do a drawing, as opposed to a diagram, and I will struggle due to poor art skills.
My drawing skills are more to do with the fact I have mild dyspraxia than my ability to visualise. My handwriting is pretty atrocious too for similar reasons. My visualisation skills are far from perfect but still much better than anything I can draw.
