Go to Settings > General > About
Find Carrier Lock or Network Provider Lock
If it says No SIM restrictions, the iPhone is unlocked
If it shows a carrier name or SIM locked, the iPhone is locked
Insert a SIM card from a different carrier
If the iPhone makes calls and uses cellular data, it is unlocked
If you get a SIM not supported or invalid SIM message, it is locked
Check with your current carrier
Ask the carrier to confirm the phone’s unlock status
Use the iPhone’s IMEI with an unlock status checker
Contact Apple Support for device information verification