I don't dream very much, though a few times a year I will have a very vivid dream (basically like a video in my head with lots of detail). This makes it very apparent that my normal imagination is lacking detail. It feels a bit like it's a different area of the brain that I don't typically use, and makes me wonder if I could use it more with practice.
One test for aphantasia goes something like this:
> Imagine a ball sitting on a table. You reach forward and push the ball and it rolls off the table. Now without changing the scene, what colour was the ball? What about the table?
For me the ball is smooth an white. The table is something akin to stainless steel. My wife will describe a colourful beach ball on a wooden table with lighting and shadow and describe the room it's in in detail. My scene was in a void as no room was mentioned.