See through mesh from opposite side of normals

How do you setup Blender so that you can see through the opposite side of the normal, while the side of the normal is visible?

For instance, you have a room (a cube, with the normals inside) and you can see through the walls from outside (because its the opposite side of the normals)?

I’ve seen this done in a blender source file but not sure how it was done.

Use Textured method to display objects rather than Solid.