Here are the methods to check if your iPhone is unlocked or not:
Check via Settings
For this method, you’ll need to have the iPhone is powered on and unlocked (the other kind-enter the four or six digits code to access the phone if needed). To check your device's unlock status:
-Open the Settings on the iPhone.
-Tap Cellular Data Options.
If you see Cellular Data Network as an option, your iPhone is probably unlocked. If you don’t see it, your iPhone is probably locked.
However, there are instances in which this method isn't accurate. If you're buying a used phone, you should definitely use one of the other methods to confirm before making the purchase.
Check with the SIM card
This method requires you have two SIM cards from different carriers. If you don’t have two SIM cards, you could borrow one from your friend.
-Remove the current SIM card and place into another SIM card of a different carrier.
-Then, try to make a call to any valid number.
-If you get an error message that the call cannot be completed, then your iPhone is locked. If you’re able to connect and make the call, then your iPhone is definitely unlocked.
Check with an Online Tool
There are quite a few online tools that can use the IMEI code to help you check if your phone is unlocked, but most of them need to be charged, the fee is from $0.99 to $2.99.
You can find the IMEI code as the following methods:
A. On the box of the phone, or on the sim card tray of the phone ;
B. Press *#06# on your Keypad dialing, then the IMEI Code will appear.
There's another kind of lock -- Activation Lock, also called iCloud Lock