I know this doesn't answer your question as to how to check if your phone is unlocked, BUT ...
This information allows you to unlock ANY Samsung Galaxy S4 for FREE (i'm not a ****, im not going to ask you for money to do it, i'll just tell you it's pretty easy).
Unlocking Samsung Galaxy S4
1) Dial *#0011#
2) Press Menu Button --> Back Key --> Menu Button (again) --> Select KEY INPUT --> Enter 1, press OK (This will enter ServiceMode!)
3) Press Menu Button --> Tap Back Again --> This Pulls Up Main Menu for ServiceMode --> Tap [1] UMTS
4) Select [1] DEBUG SCREEN --> [6] PHONE CONTROL
5) Next screen pick [6] NETWORK LOCK
6) Choose [3] PERSO SHA256 OFF
7) Go back to main meny (hit menu button followed by back key a few time's until option 6 says COMMON) --> Pick [6] COMMON --> [6] NV REBUILD
8) Phone will say a 'Golden Backup' exists, tap [4] Restore Back-up
Your device will now go through a series of “conditions”, where it will first appear to freeze, then get on a black screen with Blue LED on and both capacitive buttons on, after which it will reboot automatically. The whole cycle might take a couple of minutes, so be patient. Once it powers back on, it’s network unlocked, permanently.
I didn't make that up, try it, I promise it will unlock a Samsung Galaxy S4.
You're welcome
