What are the differences between the 1969 Cutlass "Sport", "S" and "Supreme". How can I tell the difference.

 

The S will have a S badge on the point of the hood and say cutlass on the glovebox door, the Supreme will have a Rocket badge on the hood and have CS badges on the door panels and say cutlass supreme on the glovebox door. The interior is a little fancier and there's more stainless trim on a supreme.
The VIN on a supreme will start with 342.
Here's a decent website about 69's, if you go to the decode page it'll help with the trim tag to let you know what you have.

http://http://www.69olds.com