To find the field API name used in any Apex Class - 

  • Go to the Metadata Search tab and enter the field's API name on the Search box, turn the toggle on for the Apex Classes and Click on the Search button.
  • The following Apex Classes using the field API Name will be displayed after the search is complete.
  • To cross validate results, you can see in the developer console "AccountSource" used in the "AccountTriggerTest" class -