What are some interesting things people should keep an eye out for when looking at different buildings around the city?

You can use a building's brick to help date the building!

If the face of the building (because, remember, the back and sides are likely Chicago Commons) is made up of smooth brick that's all the same color (like smooth red bricks), it's likely from the late 1800s. At that time, the fashion was to have every brick match, and the bricks were often brought in from St. Louis which made beautiful smooth red bricks. Sometimes these buildings will also feature pieces of unglazed terra cotta and shaped bricks (bricks with designs stamped onto them)

If the building has bricks with some more variation in color and texture it's likely from the early 1900s. By then, the fashion began to move toward each brick having a little more character, but walls were still mostly the same color family. You start to see more geometric brick patterning on buildings at this point too.

If the building is made of wildly multicolored and multitextured bricks and many different brick patterns (the Revere Park Fieldhouse is a great example) the building is from the 1920s or 30s. It was the 20s and we were gonna have money forever and they really built like it. You'll also see huge glazed terra cotta pieces on many of these buildings.

If the building is made up of HUGE bricks (like bricks that are nearly a foot long and a few inches tall) then the buildings was probably built in the last 20 years. Those bricks are called Utility Blocks and they're about as big as a brick can get and still fit in the mason's hand. They're much cheaper to build with because you need fewer of them. They're also much less appealing, because they're usually all exactly the same size, texture, and color. This is a byproduct of how efficient and controlled brickmaking has become. Remember how I said bricks are like pixels? Building with utility block is like taking half the pixels out of your monitor and making them all the same color.