OEM LSD tranny's will have "LSD" stamped on the transmission case in black ink, somewhere near the top. Sometimes this ink gets rubbed off though.

Carfax the VIN on your transmission case, if it comes back as an ITR tranny then assuming it hasn't been removed, you have an LSD.

Support the car on jack stands and turn one wheel counter clockwise. If the other drive wheel turns in the same direction, you have an LSD. If the other wheel turns in the opposite direction, you do NOT have an LSD.

If all else fails, remove the axle and halfshaft from the transmission. If you can see through, you have an LSD. If you see anything obstructing you from seeing completely through the transmission, you do NOT have an LSD.