There are a few ways to check this. First of all, and obviously, it must be a 4x4 S10. All ZR2s are 4x4 with a small exception of a few non 4x4 versions that came out in 2005. As far as I know all ZR2 S10s have special ZR2 only front inner fender wells that have "ZR2" stamped into the metal. You'll have to get down on the ground next to your front tire to look up in the well to look for this stamp.
All ZR2 S10s came from the factory with the plastic fender flares. Chevy's will be painted a grey color while GMC's are painted to match the trucks body color.
All late model North American ZR2 S10s came from the factor with 31x10R15 size tires. Look inside your door jam at the tire info sticker. If that is the size listed and everything else I mentioned matches its probably a ZR2.
All ZR2 S10s came from the factory with the plastic fender flares. Chevy's will be painted a grey color while GMC's are painted to match the trucks body color.
All late model North American ZR2 S10s came from the factor with 31x10R15 size tires. Look inside your door jam at the tire info sticker. If that is the size listed and everything else I mentioned matches its probably a ZR2.