You could look at the back edge of the driver's door opening, or possibly somewhere in the trunk. I'm not sure exactly where it's posted. But you can also use a VIN decoder, like
http://www.bmwarchive.org/vin/bmw-vin-decoder.html All you have to do is put in the last 7 digits of your VIN, and then it'll tell you a ton of stuff, including the paint code. Jet black is a 600 something code, while black sapphire metallic is 475.