To unlock the Reporting! trophy in Cities: Skylines, you will first need to open the inspector tool (the house with a magnifying glass on it at the left of the in-game menu) and press X to select it.
Zoom in on your map to a dense area until you can see pedestrians walking around. Hover your cursor over one of them and press X to bring up their information box.
Press X on the Green Arrow button to follow the person, and then press right on the D-Pad twice to the Toggle Traffic Routes View button. Press X and the world map will change to a grey colour with the pedestrians route highlighted by a green line.
The trophy will unlock shortly after you enter the Traffic Routes view.