Is there a code to tell if you have a limited slip differential ?
 

... anywhere on the car or transaxle ?

Depends on the year- I know that '86 onwards (could be even earlier) have a white sticker under the crest of the bonnet showing the options on the car. I believe the code is 220 or 221 for LSD.

I'm a bit of a newbie to Porsche, but I remember on the old muscle cars you could jack the rear end up and spin one of the wheels. If both spun the same direction, it was a limited slip, but if they spun in opposite directions, it was an open rear end. I suppose it would be a similar check with a 911/930 ? Doing a nice burnout is another way to tell, but it's not such a good idea with a Porsche.

LOL, good luck getting your 930 to burn out without burning the clutch. Sticker under the crest wow just when I thought I knew all the idiosyncratic ways of Porsche.

Jack it up and spin the wheel... its the same o'l story!

+1 Under the front hood is a large sticker. It'll list various codes. 220 is the one for an LSD. At least it was by the '86-'89 years (I see you have an earlier MY).

For example:
http://i5.photobucket.com/albums/y18...g/DSCN7524.jpg
http://i5.photobucket.com/albums/y18...g/DSCN7523.jpg

M220 should be the code for 40 percent locking diff. for 1978-1993 cars

220 and 222 are also the codes for the LSD and TC for the 99 996.