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 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 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.