Lift your rear end off of the ground so your tires can spin. If you turn one tire & the other one spins the other way, there is a good chance that you have a limited slip. If you have a True Posi like a mini spool or a spool when you spin one wheel one way the other one should spin the same exact way.
Now certain types of Lockers will have the low speed characteristics of a Limited slip by spinning the tires in opposite directions, but under power they should pretty much make equal length burnouts. Or you could just remove your inspection cover & that will tell you whatchu got.