You can use the gear ratio to figure it out.

Set it somewhere where you can get a good look at both the input and output shafts. Mark the very top position on one output shaft and the input shaft. Spin the input shaft exactly 4 times, arriving back at the top position. If the output shaft has stopped short of a full rotation, it's a 180sx diff with a 4.083. If it has gone past one full rotation, it's a J30 with a 3.90.