How to check mobile data balance in airtel?

I reckon the easiest way is the Airtel Thanks app. You just open it and your data is right there on the main screen. Super simple. If you don't want the app, you can dial a code on your phone. Try dialling 121# or 123#. One of them will definitely show you how much data you have left. It usually pops up on your screen straight away. Hope that helps you out, mate.