Your picture doesn't look like a shelled cube but instead like five thin blocks put together to make a cube shape. If it is a block that's been hollowed out with a Shell feature and the shell is to the inside, you can use the faces property of the ShellFeature, and it will return the five created by the shell, which are the inside faces.
You can also iterate over the surfaces and get their normals. The normals of the outside faces will be the same as the inside faces. for example, the normal of the left inside face will be the same as the right outside face, but since you already have the right face, you can eliminate it from the list, and you'll be left with the inside faces. You could also traverse the topology of the solid to get to the inside faces. It's really a bit of a puzzle to figure out something that will work and then continue to look for something that is either simpler or more efficient, or ideally both.
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Brian EkinsInventor and Fusion 360 API Expert
Website/Blog:
https://EkinsSolutions.com