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 -

PS: View Metadata Search, Salesforce quick search app on Salesforce AppExchange: https://appexchange.salesforce.com/appxListingDetail?listingId=a0N30000009wgF7EAI