There are basically three ways to check your Airtel data balance- MyAirtel app, on web and lastly through USSD method. To check your data plan using MyAirtel app, first download the app and login using your Airtel number and one time password. Tap on your account and you will see options like account data balance, your past recharges and other offers from Airtel.
To view your Airtel Plan on web, go to this link
and login using your Airtel mobile number. You can see your balance and data related info under the Account Overview tab. Further recharge can be done from the browse packs section.
Last is the old USSD method in which you need to dial *121#. The USSD when run will show several options like my offers, talktime offers, data offers and more. Select the option my number option and it will show the balance and validity of your current plan.