Some people like it to call it the “Mind’s eye” - Basically you can create images in your mind by just thinking about them. Like if I wanted to “see” a golden retriever, I can just imagine one because I’ve seen them before, so my mind knows what to do. You can also manipulate those images; so for example instead of the golden retriever being gold, I can imagine it in a purple or green color, etc.

A good analogy is that the human mind can have it’s own internal stable diffusion or Midjourney, complete with the ability to give it a “prompt” i.e. thinking about what you want to imagine.

I was surprised to learn a while ago that not everyone can visualize things in their mind. As far testing for it, I don’t really know how that would work. I don’t think you really need to test for it, I assume most people who can visualize things in their mind know that they can.