The rqt framework allows us to visualize various aspects of the ROS inner workings. Here we will configure it to show the ROS topics published/subscribed by the nodes.
Open rqt:
cd zero-to-slam/
./scripts/run-rqt.sh
Select Plugins > Topics > Topic Monitor
The window will change to the Topic Monitor:
Be sure to check the checkbox before the topic name to monitor topics as new data is published.