Apple iPhone Network Checker.
 
Use this service if you're not sure which network your iPhone is locked to.
 
We only need your IMEI number. To obtain your IMEI number, go to your dialler, and type in *#06#. We only need the first 15 digits, and the IMEI number usually starts with '35'. Please DO NOT obtain your IMEI from the sim tray, or from the housing, as these may have been changed.