If you are using Aircel Prepaid SIM Card for your mobile handset, you can easily check your current balance, talk-time available on your handset by typing *125*1# and press Ok button and now you will be able to view/see your current balance on your mobile screen. My friend bought Aircel Prepaid SIM Card recently, when he checked balance and validity, it says Bal 12.50 Val 11 Oct 12 meaning he has only Rs.12.50 balance and talk-time, validity is October 11, 2012. Tata Docomo and Idea are offering Life Time Prepaid SIM card validity.
You can also dial *133# for checking balance and validity.