flip your disc over and look on the bottom. its written in the mold as raised lettering.
the ROC is either manufactured in Ranchero , or San Marino, California.
99% of basic DX Rocs you find will be Ranchero
most of the only San Marino Rocs you will find now are the star plastic ones they are making.
Ranchero = starts overstable
San Marino = starts neutral
Ontario = starts slightly understable
http://www.discgolfreview.com/forums/posting.php?mode=quote&f=5&t=24640&p=389848
keep in mind that you can beat in a Ranchero Roc to fly like a San Marino or Ontario.
someone on here will likely correct my post or elaborate in much greater detail. everyone loves a well seasoned roc