According to the model (such as: MC536LL / A), you can find out whether the sales area (such as the National Bank, the US version, etc.) is locked or unlocked. This site Apple serial number query The results are for reference only!
Example result b>
Device status: sim-locked
Device Name: iPhone4GSM16Gb
Note: United States (AT & T1, Verizon1, Sprint
Remarks: 1- You can purchase unlocked iPhone through Apple Store