Get under the car and check chassis rails and sills, don't be afraid to give them a proper poking about. Ensure it doesn't have a pair of nuts of the front suspension turrets as this would mean it's an older shell and not genuine S1, also check throughly round the ID numbers to ensure the car is genuine.

I find the best approach is to assume the seller is up to no good and try and prove the point. It's steered me away from many a polished up shitter in my time and saved me thousands of pounds