The only way of telling that I know of is with the drive shafts and the output shafts removed look through the hole the output shaft came out of and if you see a pic going across the centre of the hole then it's and open diff. If no pin can be seen then it's a LSD
bring her into courtown carpark at about 40mph push in the clutch pull up the handbrake lock the steering put the throttle to the floor and let out the clutch
if it does half a donut and catches grip its not an lsd