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 Network & Internet.
Press Mobile network.
The total data usage is displayed.
Press App data usage.
The data usage for each application is displayed below the name of the application.
Slide your finger upwards starting from the bottom of the screen to return to the home screen.