Simply check the rear frame rails as viewed through the rear wheelwells. If both rear frame rails have the factory welded doubler plate for the exhaust hangars it's an SS. Only cars with dual exhaust came with doublers welded onto both frame rails. SS and z/28's all had to have these. Also the SS frame stands for the engine mounts will have oblong holes, not round. Grille should be black, not painted black, molded black. Check your glove box tire pressure decal also. Check what size fuel line you have. All 69 SS had quarter panel louver trim, chrome accents on the tail lights, roof drip edge, ect. Pull the vent out of your door jamb and look inside the rear quarter to see if the four holes have been filled where the louvers once were. There are lots of other small details to look for as well, these are just a few of them.