How to tell if your iPhone is AT&T, Verizon, etc.

"Turn it over. If the model number is A1428, then it is an AT&T or T-Mobile phone. If it says A1429 then it is a Verizon or Sprint one

IMEI or MEID is available on lock screen. IMEI is for A1428. Enter the IMEI or MEID in the carrier website to determine the carrier"

"Here is a complete list from Apple on all iPhones and model numbers.

https://support.apple.com/en-us/HT201296"

Actually, in most cases, it is impossible to tell which carrier the iPhone is or was used on by simply looking at the model number. Though in some cases it might work. There is actually an IMEI check which can show what is known as the "Initial Activation Policy" and this would be the original carrier of the device. Notice however that the same device can be used on another carrier later.

To find what carrier network it's locked your iPhone, Verizon, T-Mobile or AT&T, you can do this using iPhone IMEI Checker service. You need to find your IMEI number from your Cell Phone, and go here iPhone IMEI Carrier Checker to find the full informations for your device.

If like to find what carrier network it’s locked need to use some official IMEI checker from AT&T or Verizon company. Then can see. Or if not know need to use GSX Apple IMEI checker