How do you know if it is a Saab spg?

First: it probably isn't. Saab only ever made about 7,000 of them. And all the decorative and "exterior performance" items, like the wheels and the side skirts, that were on the SPG cars could also have made their way to a regular 900.

The only way to tell if you have a real, genuine, Saab 900 SPG is to pop the hood and look for the Automatic Power Control box - the device that keeps your car from blowing up if you put too-low octane gas in it. On ordinary Saab Turbos, the APC box is black. On an SPG, it's red.