There's a setting in the graphic details - "show hidden items" or something like that. If you combine that with the cameraman mode, it's not hard at all
Walls up/down can be controlled by home/end on your keyboard, and floor up/down by page up/page down. While in cameraman mode, the camera won't jump to the next floor but stay put, so you'll see all the floors that fits into the screen.
I don't think it's possible to do this properly with the play mode camera, so it's mostly useful for screenshots or such.