The available Z71 Off-Road Package includes:
•Z71 Off-Road suspension
•46mm twin-tube high-pressure gas-charged shocks
•Skid Plate Package with a frame-mounted shield
•GM segment-exclusive automatic locking rear differential that provides added grip when you need it
•Available Z71 Appearance Package: 18" aluminum wheels, body-color mirrors and door handles, body-color grille with chrome insert, and Rancho® shocks
If the RPO sticker is readable in the glovebox it will show a "Z71" on it. Although you may as you say have a metal plate on the firewall listing the build options.