The Arduino code editor has a serial monitor which can be used to view data sent by your robot over a USB connection to your computer.
Connect Robot to Computer
Connect your robot to your computer using the USB cable. If necessary, upload your app to your robot.
Two important reminders:
If your app will be driving the robot's motors, be sure the robot is standing upright on its back end (so its wheel are in the air) to prevent your robot from driving away while it's connected to your computer. (Another option is that you could temporarily set the Motor switch to STOP.)
Do not unplug the USB cable. You'll need to keep the robot connected to your computer, in order to allow and view the serial data communication.
View Serial Monitor
Arduino Create Web Editor
Click the Monitor menu link in the left navigation to display the serial monitor in the middle panel.
When you are finished viewing the serial data, you can click the Disconnect button.
Arduino IDE Desktop Editor
Under the Tools menu, select "Serial Monitor." A new window will appear displaying the serial monitor.
When you are finished viewing the serial data, you can close the serial monitor window.