You can see how much mobile data you've used when you've used the browser or sent and received email messages etc.
Slide two fingers downwards starting from the top of the screen.
Press the settings icon.
Press Wi-Fi & Internet.
Press Data usage.
The total data usage is displayed.
Press Mobile data usage.
The data usage for each application is displayed below the name of the application.
Press the Home key to return to the home screen.