how to check how much data used

Have U try https://www.o2online.de/vorteile/apps-zusatzdienste/mein-o2-app/

Hey ​@Mahimamaria,

Welcome to our o2 community. We're glad you're here.😊

You can register with your contract in our app, for example, and view your consumption there. Schluej has already sent you the link. Feel free to give it a try.