When I "see something in my head" while dreaming, it is like seeing: There is a clear visual image. I don't see the black behind my eyelids. I can tell colours. I can tell shapes.

When I "imagine" something while I awake, it is not remotely like seeing. There is no image overlaid on the "real world" whether obscuring the black behind my eyelids or anything else when they are open. I can't tell shapes, or colours. If my eyes are closed, I just see the inside of my eyelids, nothing more.

My one experience of seeing something while awake during meditation was like being inside a movie. Everything was as clear as if I had been walking around in that setting for real.

I don't know what it will take to make you sure, but for me, I have experienced three entirely different "modes" of imagining things, ranging from no visuals, to shifting and unclear visual, to crystal clear images, and the distinction is stark and obvious - there's no ambiguity in it.