1 You can also find out if you’ve won by entering you Premium Bonds holder’s number into the prize checker. 2  Your holder's number (also known as a customer number) is the number that connects you to all of your bonds. If you have moved and not informed the NS&I then you could have won without realising. 2  You can also find out if you’ve won by entering you Premium Bonds holder’s number into the prize checker. 3  Your holder's number (also known as a customer number) is the number that connects you to all of your bonds.