Every mobile phone comes with unique code called IMEI number. This number is unique for every mobile phone. This number can be used to uniquely identify your phone. If Your phone gets stolen, You can block your phone if you know its IMEI number.
IMEI number is printed on box of phone. It is also printed on purchase receipt. To know IMEI number of particular phone dial *#06# .
But what if your phone is stolen and you don't know the IMEI number. To stop someone from using your phone you need to block it. But for that you need IMEI nuber.
Find Out IMEI Number :
When you register your phone with play store on android phone , Your IMEI number gets stored on your gmail account which you used for registering your phone on play store. So to find out your IMEI number, sign in to your gmail account and go to https://www.google.com/settings/dashboard . Under this setting there is Tab named android. Expand it you will see all the IMEI number of android phones associated with this Google account.
See Screenshot:
This way you will be able to know your IMEI number of lost android phone.