How to check if my 5s is really unlocked?

[graylocks]

i'm preparing my 5s for resale. AT&T ok'd unlocking it and sent me the instructions which were to back-up, erase, and then restore. i did that and from postings here i expected to see a Congratulations you're unlocked message but i didn't. i'm thinking maybe i should have just chosen restore rather than restore from back-up. yes? no? i've already taken its sim card out and put it in my new 6s. if i erase and restore the 5s again does the sim card need to be in for the unlock procedure to work?

i'm staying with AT&T so i don't have another providers sim card to put in to see if the 5s is unlocked.