You might buy a second hand iPhone, and you want to make sure that iPhone is the locked or unlocked one? Check the imei number to clarify the matter.
How to get the iPhone imei number?
Setting > General > About > scroll down te see the IMEI code.
Check the iPhone is Locked or Unlocked (World) version
Go to http://imeidata.net/iphone/unlock-check, enter your “IMEI code” and click on “Check”.
This is the unlocked one (world)
This is the locked version