There will be a plaque in the interior stating what stage/variant it is. There will also be a sticker in the passenger door jamb stating the production number of that particular year. As Ros said the last 5 digits will give all the info you need. Every Roush is entered in the Roush registry before leaving their facility. Only the registry is a true way to tell if it's a Roush. Being you cannot replicate or enter the vehicle into the registry. Unlike adding all the Roush mods/graphics/plaques/window sticker. Which can all be replicated but the registry prevents people for trying to sell Roush clones off as genuine Roush vehicles. On Roush's website they offer a free check for VIN verification.
Unfortunately the car will not be covered as a Roush but only insured as a Mustang GT (donor car) in most cases. There are some Roush Cobras out there. Unless you get a modified vehicle coverage your insurance will not cover the Roush modifications. Due to the fact it's a tuner car. One insurance, Hagerty, will insure the car as a Roush mustang, and cover all mods done by Roush to the vehicle. There might be one or two more but in my opinion Hagerty is the best.
Every day is a joyride.