Originally Posted by JazzM
The diff housing usually will have the ratio stamped right on it...
and its easy to tell if its an LSD.
BTW, you will also be able to use the e32 (7 series) and e34 (5 series) diffs (possibly the e28 as well but not 100% sure) if you want to get a specific ratio without rebuilding. be sure to check that its an LSD.
BMW motorsport also has rack and pinion gears available if you want to pick up the gears and just swap those out from yours.
E28 diffs are 100% interchangeable (just need to reuse your original rear cover and output shafts), however E32 and E34 units are not (to my knowledge).