Answer to "Can I change an imei number"Yes, it’s possible to change the IMEI number on any cell phone models. To do this it’s possible only using Box Tool. This tool depends on the device phone model. For LG is Octoplus. For Samsung, you have Z3X Box Tool. You have here full guide and tutorial how to download this software for free and full instructions on how to use https://www.unlockbasecodes.com/how-to-change-imei-number-using-box-tool-for-free/ very easy.
Answer to "How to tell if your iPhone is AT&T, Verizon, etc."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