There are three ways to check prepaid balance on Airtel. The first is through the MyAirtel app, the second one is through the web, and the last is by dialing *121*13# - read on for detailed instructions.

source https://gadgets.ndtv.com/telecom/guide/airtel-prepaid-account-balance-plan-validity-how-to-check-app-ussd-code-guide-2378576#rss-gadgets-all