It couldn't have been a factory RS Z/28 without the X33 tag, so that is a good sign, it's possibly original RS. But likely the car has been restored at some point if so, the tag looks to have been masked and painted around, and it is missing the rocker spears, wheelhouse/fender lip trim and the quarter louvers that an X33 car should have. The driver's side forward driprail trim has slipped down or been installed too low too, plus the vinyl roof has been added as you noted. Fender/hood/cowl alignment needs adjusting. Minor stuff, save for removing the vinyl roof; but I'd leave it, it looks good and breaks up the Frost Green nicely IMHO.
If you know it, check the VIN falls within the correct range for that build date. Also there are many clues to look for that can provide hints to the car being a genuine RS/Z (or not), lots of good info on this forum and the CRG articles. Firewall piercings and stamped backup light holes are a couple of examples of original RS details to look for, and for a Z, look for the axle assembly code and partial VIN on both the block and transmission.