Safe-T-Track is the name Pontiac used for it's limited slip differential. Some other brand's names are:
Twin grip;
Twin Traction;
Traction Lok;
Power-Lok;
Positive Traction;
Sure Grip;
Positraction.
Most just call them Posi or limited slip.

As Bob says, lift both rear wheels off the ground and with the transmission in neutral and the park brake off, turn one wheel by hand and if the other wheel turns in the opposite direction you have a non posi.
If they both turn in the same direction you have a posi.
Check that vent while you're at it if it's plugged you could build enough pressure in the axle housing to cause an oil leak. Been there, done that.