How do I tell if my AT&T iPhone 5 is unlocked?

I requested AT&T to unlock my iPhone 5 at http://att.com/deviceunlock . I got an email saying that my device may be unlocked. According to the instructions in AT&T's email, I need to do an iTunes backup and restore to complete the unlock process. I have NOT done this yet. I bought this T-Mobile SIM and inserted it in my iPhone and T-Mobile shows up in my phone's status bar. I cannot make phone calls because I have not signed up for T-Mobile service and have no intentions to. So, is my phone unlocked at this point? Is there something else I can look at to confirm this? Thanks!