Before u insert credit card no. u have to select one kind 
of bank(HSBC,HDFC,SBI,AMERICAN EXPRESS,AXIS,KOTAK 
MAHINDRA,ICICI...)or else u should select for visa card or 
master card american express card.Here the main thing is 
the third party softwares are comes into picture.suppose if 
u select HSBC credit card then our application should ask 
help of HSBC.The developers are interfaces these third 
party softwares where exactly they required.

There will be two level of verifications:

1. Front End  - Where first we will check whether user has
entered valid card number or not. Normally in India Credit
Card is 16 digit Numeric. So front end need to check if
digit entered is 16 digit and its numeric only, if its not
throw error there only invalid number.

2. Back end - If number entered is 16 digits, we will be
verifying it against the database of bank. so if its correct
then continue with transaction or else throw error Credit
Card Number not found.