how can i determine if the rear differential is positraction or lsd

easy way is to jack the car up, turn a wheel and look at the other one. If it turns the same way, (I beleive) you have posi, if the wheel turns the other way you have a floating diff. hope this helps

Yes, you can also check the axle code on the door information sticker with the tire info, or have a dealer decode your VIN for you at no charge.

dont look too hard for a posi diff from an 88 300zx because they only came with a clutch type lsd, and no an lsd will spin both tires the same way, an open one wheel wonder diff will spin them opposite

Nissan was very good about identifying what was contained in the differential by indicating so on the differential housing.  The 1988 differential was coded R200 (200mm ring gear).  If it had a factory LSD then it would be stamped on the housing.  If so then the LSD was a clutch type LSD.  I believe the Z31's also had a LSD badge on the rear if the car came with an LSD.  There was never a locking-type factory differential on any of the Z-cars.

For added info the Z32 300ZX's had an R200 with open diff in N/A trim and the Twin Turbos had the R230 with a viscous-type LSD.

FYI posi and lsd are the same thing. posi is what dodge called a limited stip in the 60's

Great answers by everyone else, another quick way to check... if the diffrential has a finned cover it has the clutch type LSD.

Years:

1987-1989 300zx Turbo - Clutch type LSD
1988 300zx Shiro Edition - Viscous type LSD

just get in ur car and take a corner on the gas and see if u chirp of slide if it does this it's posi