You mean querying the credit of a prepaid card? The info isn't stored on the card, it has to be asked from the provider by a CUSD message. The service code is provider dependent, the same code you type to a mobile phone, e.g. *100#. The respective AT command would be AT+CUSD=1,"*100#"<CR>. It's answered by a CUSD unsolicited result code and text message telling the credit balance.