Posters
One of the easiest ways to spot the hidden messages, references, and throwbacks peppered throughout the films and episodes of the MCU is to take a look at the pictures, photos, posters, or billboards in the background of the scenes. It can take a while for viewers to spot these, and often it happens on the second or subsequent viewings.
For example, in Spider-man: Homecoming, we can see rather a subtle nod to Mark Ruffalo’s Hulk. Above the whiteboard in his science classroom, Peter Parker can see pictures of famous scientists throughout history, including, at the end of the row, one Bruce Banner.