There are two methods on how to check if an iPhone is SIM unlocked (or carrier unlocked):

Method 1

• Ensure iPhone is on latest iOS then:
• Go to Settings – General – About & scroll down to “Carrier Lock”.
• If unlocked it will say “No SIM Restrictions”.

Method 2

• Open SIM slot using a pin or paper clip.
• Insert a different carrier's SIM card.
• If iPhone is SIM unlocked it will auto activate and you will be able to make calls and use data to browse the web.
• You should also be able to double check by going to Settings – General – About & scroll down to “Carrier Lock”, if unlocked it will say “No SIM Restrictions”.