Full or hollow, plaster, cellular concrete, concrete, limestone, brick... Not always easy to know the real nature of a wall. The solution: pierce!
And by observing the dust of the borehole, one immediately identifies the material that composes it.
If the drilling dust is ocher or red, no doubt: your wall is built with bricks.
The dust is fine, white or gray in color and it runs off: it is a concrete wall.
The dust is beige or white and sandy: it is a wall built of limestone.
The dust is white, coarse and slightly oily: it is a wall made of cellular concrete.
The dust is white, fine and "glue" to the drill: it is a wall made of plaster (tiles or plates).