Guide to checking IMEI of an old iPhone?

Specifically, I've heard people say that before buying an old iPhone, it's necessary to check the IMEI to see if it's a locked or unlocked version and some other important information. Do any of you have experience with this and can you guide me on how to check the IMEI of an old iPhone?

Thanks a lot!

Check out this link for a guide on checking the IMEI of an old iPhone: https://Mytour.vn/xin-huong-dan-check-imei-iphone-cu-2687n.aspx