The quickest visuals are:
1 - The rotors are slotted and vented, see attachment. Non-Z51 has solid rotors.
2 - The 19 and 20 inch wheels, see attachment, are unique to that option, but were also available in the "ZF1" appearance package.
3 - The car will have the dry sump oil system.
The most accurate indication will be the Window Sticker and / or the build sheet / options list located in the rear cargo compartment, under the carpet. Or in the trunk, under the carpet on the convertible. If it has that option the RPO will show "Z51"
There are a number of other identifiers but these are the most conclusive.
Hope this helps
Ross