how to check my phone is unlocked

Try a SIM card from another carrier and see if it works.

1# Make sure you have an internet connection in iTunes.

2# An AT&T Real MicroSIM(not one that has been cut down) must be in the phone.

3# Restore in iTunes.

Hope this helps!!

Go to http://iphoneoperatorcheck.com/ and enter your IMEI number (SN works fine too). Note that a phone which has never been activated with Apple will show as unlocked until activated. If the phone is activated and still shows as unlocked then the phone is truly unlocked.