Step 1: Find out your IMEI number by going to Settings > General > About.
Step 2: Go to IMEI.info and enter your IMEI in the box. Click “check”.
Step 3: The website will load information about your iPhone. At the bottom right of the page, click on “Free simlock status check”, as showed below.
Step 4: The website will check your IMEI against the database of factory unlocked iPhones and will let you know whether your iPhone is officially unlocked or not. Additionally, it tells you about the warranty status of your device.
The whole process should take you just a few seconds, assuming you have your IMEI