....take a turn hard, stomp on it in 2nd. if the rear inside wheel spins then its not an LSD, if the car accelerates or starts to go sideways you have an LSD;)......or you could jack up the rear end and spin one rear wheel while having a freind hold the other and see if the one that your freind is holding is trying to spin, if so then you have an LSD.
or you could go with option 3, and do what ak96ss said.