For me I can "see" things in my minds eye and it almost doesn't matter if my eyes are closed or not. The detail isn't perfect and it's unlike anything else I would ever "see", such as a blurry image or simple drawing. I can manipulate the image in my mind, rotate it or fold it over, etc.
The longer I think about a specific part the more detail I can see in that part. Unlike when I look around and see infinite detail all at once, my minds eye only sees the detail when I really focus on generating it.