Apple GSX Report
When you're considering whether to buy a used iPhone, or trying to unlock one, running a GSX first will help.
Sample Report You'll Receive:
Serial Number: F2LWFW1FJXXX Initial Activation Policy ID: 2387 MEID: Activation Policy Description: US Verizon Locked Policy IMEI: 353013096990XX9 Applied Activation Policy ID: 2387 Part Description: iPhone 8 Plus Applied Activation Policy Description: US Verizon Locked Policy Product Version: 11.3 Next Tether Policy ID: 2387 Last Restore Date: Next Tether Activation Policy Description: US Verizon Locked Policy Bluetooth MAC Address: First Unbrick Date: 04/15/18 MAC Address: 645AED415XXF ICCID: 89148000002911426XX6 Last Unbrick Date: Unbricked: Unlocked: true Unlock Date: 05/05/18 Find My iPhone: OFF
Why Unlocking Matters
From the article linked below:
"It’s simple. Once unlocked, you can use an iPhone (models that are GSM devices with SIM cards) with any mobile carrier around the world providing GSM service. Just remove the current SIM card and pop in the new SIM card from your carrier of choice.
That’s why it’s so important to perform this vital check before any purchase. You should be 100% confident that the iPhone or other device is indeed unlocked.
Here's the article quoted above.