Each SIM card has a unique serial number or S/N. M2M One dispatch a packing slip with each order that lists each SIM with the serial number(s).

The barcode on the SIM cards sleeve is the serial number, and some sleeves/SIMs have the serial number printed on them, i.e. 0000 1111 2N, which means that the S/N is 0000 1111 2 without the N.