[Freeswitch-users] Is this an internal call?

This must be a FAQ, but I can't find it. In the default dialplan, how can I tell if the call was from an internal phone, or from an external call that was transferred? One page said there was a variable ${internal_call}, but I can't find that anywhere in the source. Should I just look to see if a directory variable, like account_code or user_context is set? -- Steve