There are lists of exterior visual clues that help you determine if itís a Z28; however, these items can also be "clone". In addition to the item others have noted, also check:
1.) Front disc brakes with the brake pressure switch "manifold" located under the master cylinder, and the metering valve located on the bottom right side of the master cylinder. Also look for the pressure regulator valve along the outside frame rail, under the driverís door side of the car. These items are not unique to Z28 but were include on them.
2.) Single 3/8" fuel line front to back with the original looking fuel line clips.
3.) The reinforcement steel plate welded to the driverís side rear frame rail for the duel exhaust hangers.
4.) Check for evidence if the car originally had an automatic transmission. Look how the speedometer cable routed. Automatic trans (and the manual three speeds) ran the speedo cable out the RH side (driverís side) of the trans up under the driverís side front floor board between the floor board and the frame. It runs along side the emergency break cable and retained with an olive green clip. Automatics use the "production speedo cable hole" located near the bottom LH side of the firewall, just lower left of where the brake booster/master cylinder attaches to the firewall. (See p. 330 in the AIM). Also look for a large 1 -1/2" hole in the forward area of the trans hump in the floor board where the automatic shifter cable with grommet was run thru. The Muncie 4-Speed speedo cable exits the RH side of the trans and is attached to the RH side of the floor board and routed across the lower front of the firewall, in the trans tunnel area and enters the interior on the drivers side area of the tunnel near where the tunnel blends with the vertical face of the firewall (See hole piercing info in the AIM on page 9). The speedo cable is secured to the firewall with three clips P/N 3772324 (see page 330 in AIM). Try to look at both 4-speed cars and automatics and you will clearly see the difference.
On other more sure way to see if the "Z28" is real and if it has not been totally restored, is to look for paint pin letters on the rear bulk head sheet metal, behind the back seat and under the card board panel and rear package tray that seals the trunk area from the interior. Many RS Z28's will be marked "X3" for the X33 option code and standard non-RS Z28's will be marked with "X7" for the "X77" base Z28 model. This is a though one to check out without taking the car apart and potentially upsetting the owner. If the car has been totally restored, pictures of these markings may be available to see.
Again, most all of these items can be replicated but some quick clues to look for.
Good luck with your search!