In various scenes in Star Trek TNG will can see out of the windows. The Observation Lounge has a whole wall-full of windows. Then - why can you NEVER see other parts of the ship through the windows?
From the observation lounge, immediately behind the Main Bridge on Deck 1, you should see the top of the saucer section sloping gently away, the top of the Engineering section, and the nacelles. But you see none of this. Only space.
Another area is the shuttle-bays - with the door open, you should be able to see the external top of the saucer section just outside and the nacelles, but again, it's always just space.