Here is the procedure to check your 3G data balance. So that you can keep an eye on your data usage balance there by leaving away the chance of losing money from your main balance.
Please let me know through your comments if there are any mistakes in the list
Aircel: Dial *111*9#
BSNL: Send SMS DATA3G to 53733
Vodafone: Dail *111# and select option 6
Reliance (GSM): send SMS BBAL your mobile number to 53535
Idea: send SMS USAGE to 58880
Tata Docomo(GSM): Dial *111*1#