By Fuel Economy mode do you mean AFM/DOD where 4 cylinders are shut down? If so then yes, you can monitor the proper PIDs and see what DOD/AFM is doing.

Also, is it a NBS or classic truck? 07/08 is the switch point.

The PIDs I see in the Scan tool are the Generator L-Terminal Duty Cycle (%) and Alternator F-Terminal Counts (%).