The spoiler is optional so that's not the best indication. As mentioned before the 3 items that are totally unique are the front air dam, wheels and brakes. Of those 3 the only externally identifiable option that can be counted on are the slotted brake rotors as they are the most likely not to be retrofitted.Stealthman
Not sure about markings. But if you're trying to identity a Z51 from the exterior, the rear spoiler is full length.
In addition, Chevy has a dealer optional Z51 spec wheel that looks a lot like the base wheel that came on the Premier edition so although the wheels are the quickest indication they cannot be relied on.I would add one thing: not all wheels (rims) on the package have to be black ; I have the Z51 package and ordered it with polished chrome wheel option.