Go to Settings
Tap General
Tap 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 phone works and gets service, it is likely unlocked
If the phone shows SIM not supported or similar, it is likely locked
Check with your carrier using the IMEI number
Use Apple Support or your carrier account to confirm unlock status