Originally Posted by kent370

To determine if you have the Sport package, check the tire sticker in the driver's door jamb. (see attached pic).

This sticker tells you what tires your car was equipped with when it left the factory.
Here in the USA, the only way to get 19" wheels/tires from the factory on a 370z is to order the Sport package.

So:
If this sticker lists 19" front/rear tires (as in "275/35R19"), you've got a Sport package car with the limited slip differential.
If it lists 18" tires front/rear, you do NOT have the Sport package, and do NOT have a factory limited slip rear axle.