it will be printed on the VIN plate.. 1st line is your chasis codes, etc.
2nd line is the drivetrain configuration, gearbox and diff ratio
I forget the exact configuration of the numbers, but if you see RP43 is 4.3:1, RP39 is 3.9:1, etc.. Memory is a bit fuzzy, and can't recall off the top of my head right now, but you should be able to work it out.
my r32 auto is 4.3:1 btw