CrystalExplorer can visualize the "empty" regions of a crystal structure.
In CrystalExplorer the void surface is defined as an isosurface of the procrystal electron density, and calculated for a whole the unit cell. The default isovalue is 0.002 .
Where the void surface meets the boundary of the unit cell, capping faces are generated to create an enclosed volume. These caps can be turned off by unchecking the menu option Show Surface Caps in the Display dropdown menu.
As well as visualizing voids, CrystalExplorer also reports the volume of the void and it's surface area (which does not include the capping faces).