Actually it is not that complicated to determine which version is locked or unlocked
For iPhone 5 or 5C or 5S that bought in Apple Retail Stores:
AT&T version:
LOCKED for contract; UNLOCKED for device only purchase. For locked iPhone, you can request unlock if you have fulfilled the contract.
T-Mobile USA version:
UNLOCKED
Verizon:
UNLOCKED for BOTH contract and device only because of the FCC requirement of the 700MHz spectrum that Verizon is using
Sprint:
LOCKED for BOTH contract and device only. You can request unlock ONLY for international travelling if your iPhone is attaching to an active Sprint account. (i.e. only non-US carriers sims can be used even it is unlocked. AT&T, T-Mobile USA and other US carriers sims WILL NOT WORK)
For iPhone 5 or 5C or 5S that bought directly from carriers or authorized resellers (including Best Buy, Wal-Mart
etc)
AT&T version:
LOCKED for BOTH contract and device only purchase. However, you can request unlock if you have fulfilled the contract OR your iPhone was purchased as device only (or called no commitment pricing or without contract or full pricing). You will have to provide proof of purchase (e.g. store receipts) if you want to unlock for iPhones bought without contract.
T-Mobile USA version:
LOCKED. T-Mobile USA will unlock it only under certain requirements. Most common requirement is the phone must be active under T-Mobile account for at least 40 days.
Verizon:
UNLOCKED for BOTH contract and device only because of the FCC requirement of the 700MHz spectrum that Verizon is using
Sprint:
LOCKED for BOTH contract and device only. You can request unlock ONLY for international travelling if your iPhone is attaching to an active Sprint account. (i.e. only non-US carriers sims can be used even it is unlocked. AT&T, T-Mobile USA and other US carriers sims WILL NOT WORK.