does the hood extension (the piece that juts down between the grills, yes the one that you will bang you head on eventually) have a trim badge that has the "olds rocket" in it, or does it have a badge with the "S", the supreme has the "rocket". The front fenders on a supreme would only have a badge that says cutlass, the "S" model has a seperate badge underneath the word Cutlass that has an "S". The door panels on a supreme have a badge that says "CS" Also, like Joe said, Supremes have a unique vin #.
Here's some links:
www.fusick.com
www.yearone.com
www.opgi.com
There's plenty of others on the web, look around.
Good Luck
Tom