How about this - Picture an elephant sideways. Don't let the elephant move. Is the trunk on the left or on the right side? If they can't answer, then it's possible they have Aphantasia.
That's easy to answer without images. Either by reasoning your way out of it, or more plausibly, with this kind of 'geometrical imagery' that comes instead of images. I 'see' a sort of figure-stick elephant and can tell you immediately where the trunk is. Maybe a test involving details of pictures would fare better?
