If it IS a wavetrac, you can easily tell by jacking one rear tire off the ground (use a floor jack and orient its wheels such that the car can roll forward) and then carefully give it a SMALL amount of throttle. If it is a wavetrac, the tire on the ground will still propel the car forward. A stock diff will just spin the tire that is in the air (as well most other non-wavetrac LSD diffs). To test if it is a non-wavetrac LSD (Quaife, for example), try putting one tire in a puddle of soapy water or in soft dirt and have someone observe as you do a mild " brake stand".

Cheers,
Chris