Tag whether the viewpoint is inside or outside. For example, what you can see through a window is completely irrelevant. What matters is whether the viewpoint is inside or outside.

If the background is too ambiguous to tell (example: it's a pool, and there's no way to tell whether it's an indoor pool or outdoor pool), you can skip.

Even if you can tell if the light source is artificial indoor lighting or sunlight, if you can't actually make out any of the objects in the background, it's likely better just to tag simple_background and/or abstract_background or skip, rather than tagging inside or outside. Keep in mind that the people searching for inside or outside are likely doing so to try to avoid simple and abstract backgrounds.