Close your eyes and try to visualize an apple. Do this for 30 seconds or so. Try to visualize the skin, the reflection, the texture, the stem, the depth, etc. Try to hold a stable mental picture of that apple.
After the 30 seconds, rate your ability to picture the apple from 1 to 5, where 1 is complete inability and 5 is as if you were looking at a picture of an apple for those 30 seconds. 1 is aphantasia.
Another idea is to recall a vivid dream you had. I think most people would describe it as being part of a movie or reality. While awake, are you able to recreate scenes in vivid detail as if you were dreaming? 5 for complete parity and 1 for not at all. 1 is aphantasia.
Everyone answers correctly the ball will roll of the table and fall to the ground. But then ask them" "What was the color of the ball? What was the size of the ball? What was the gender of the person pushing the ball, what clothes were they wearing?"
People with aphantasia are usually stunned by the follow up questions. People who don't have aphantasia really have seen the table, the material its made of, imagined a ball of certain size/type color (e.g. multicolor beach ball, or basketball or what ever), and they saw an actual person pushing the ball, they saw the ball rolling on the table an falling to the ground and can answer details about their vision.
For me the ball kept rolling off the table and rolling through air but not falling to the ground, even while realizing I should be causing it to fall to the ground, but rolling straight just "felt" natural at that moment because it's in make-believe land it can do whatever.