How can I know if it is indoors or outdoors (apart from the obvious)?
Can I change outdoors into outdoors and otherwise?
I'm mostly interested in windmills (can I make them work underground) and farmplots (can I plant cave plants outside and regular plants in the caves).
I'm assuming you have the Premium version, because in older ones the info is readily available.
There is not direct information (unlike in older versions), but: if some tile is under cover it is "inside", even if it was outside previously, and you have built a cover over it. You can build beds on this tile (unless it's a slope or flooded etc.), but when you try to build a bed outside it displays information "Must be inside".
There is also a light/dark dichotomy, where "light" cannot be changed back to "dark" even if you build a roof over it. This can be discerned on flat surfadces by the "Cavern" keyword: "sand Cavern floor" is "dark", while "sand" is "light", i.e. it was exposed to the surface, however briefly, and stayed that way.
Since windmill require "outside", and "outside" changes into "inside" if there is some floor or wall above it, you cannot build working windmill underground.
But plants planting depends on the "light"/"dark", dark allows subterranean plants, while light allows surface plants. So you can farm surface plants underground, if you have exposed it to the surface, and then covered.