Inside and outside plans. Infer the internal organization of a building from the outside setup of doors and windows. Where is the staircase? How many apartments are there? Where are the separations between apartments? Much of the building plan can be guessed from examining the disposition of the windows.
Ceiling heights. It happens that adjacent buildings have the same height yet a different number of floors: one has 7 floors, the one next to it has 8 floors, yet their roofs are lines up. Why? Because the floors have different heights. Correlate floor height to the age of the buildings.
Symmetry and near-symmetry. Find perfectly symmetrical patterns. Find patterns that are not symmetrical. Why not? Are those departures from perfect symmetry accidental or voluntary? Example: The central column of Notre Dame's facade, the tip of the roof behind it, and the tall steeple in the back, are not quite lined up, thus missing an opportunity for pleasing symmetry.