Airtel offers variety of way to check you bills. Here are the SMS codes.
Sms "BILL" to 121 for your last month bill and your current bill amount.
Sms "CL" to 121 to know your credit limit. Works for post paid accounts. It show you how much credit Airtel is giving you.
Sms "UNB" to 121 to know the unbilled amount per month.
Call *225# to know your balance and expiry date.
Call *369# to know the balance and expiry date.
Dial *102# to know last call charges.
Dial *123# to know your balance.
If you know other codes even for other mobile operators like bsnl and reliance etc comment away.