Each SIM card has unique identifier - ICCID (Integrated Circuit Card Identifier). Serial number is stored in SIM card memory and printed on SIM card body. It's required when you want to transfer phone number from one operator to another. You can pull out SIM from the phone and check the ICCID number but there is easier way - just use ICCID application.
This application allows to get quickly ICCID value without pulling SIM card. In addition it displays information about SIM card mobile network operator.
Dual SIM phones are supported if phone runs Android 5.1 or later.