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.
I was shocked to realize that when people said "imagine in your minds eye", they meant it literally. This seems to be a common experience for people with aphantasia [0].
Note that when I'm close to sleep or dreaming, then yes, my minds eye visualization is close to photographic parity. While awake, its almost completely non-existent.
[0] "I can’t picture things in my mind. I didn’t realize that was unusual" https://www.theguardian.com/wellness/2024/feb/26/what-is-aph...