ESN check from official Carriers
Verizon – https://www.verizonwireless.com/od/prepaid/bring-your-own-device/#/checkDevice
AT&T – https://www.att.com/deviceunlock/#/unlockstep1
T-Mobile – https://www.t-mobile.com/resources/bring-your-own-phone
Sprint – https://www.sprint.com/en/shop/bring-your-phone-to-sprint.html
MetroPCS – https://www.metrobyt-mobile.com/bringyourphone
US Cellular – https://www.uscellular.com/phones/bring-your-own-device
Cricket – https://www.cricketwireless.com/cell-phones/bring-your-phone
Boost Mobile – https://activate.boostmobile.com/bring-your-phone
Xfinity Mobile – https://www.xfinity.com/mobile/byod
Ting – https://ting.com/byod
Third Party ESN checker
http://www.checkesnfree.com/
https://swappa.com/esn
https://igotoffer.com/check-imei
https://www.imeipro.info/