Find it on the box or its original receipt. Other option is to find its IMEI number. It should be at the back of the phone or when you dial: *#06#. Below link will take you to the website where you enter the IMEI and you will see the name and model of your phone.
Click here.

Need assistance? Call us and we will help you to find the IMEI / serial number: 0208 543 70