Seems to me a way of testing the visualization vs hallucination (if I can distinguish it like that) gradient would be the ability to trace visualized/'projected' objects on paper, particularly for subjects without prior background in illustration/draftsmanship (less skill to lean on) and for images which require replication of details they're not familiar with (things like dice are relatively geometrically primitive shapes, while say a person's face while it can be built up from primitives is inherently complex). Granted part of this would be testing photographic recall.

Like you describe I can visualize a lot of things, with fidelity, but it's still in my mind's eye (that is, a 'sense' rather than a physically represented image that's as apparent as other IRL objects). I can also hone in on details with my eyes open while doing other things. However I can't trace such things merely from this sense since they're not actually there for me. I can however leverage my draftsmanship skills to be able to focus on the mental visualization/sense and progressively draw from memory and for various familiar things visualize them on the paper but not as an optical manifestation.