On Miatas and S2000 it takes a loooot of abuse to wear out (or usually brake) friction rings of the LSD.
Usually a lot of drifting. And i mean a lot.
At the Porsche GT3 forums i've seen complaints that stock LSD stops being limited after just few months - so maybe P's diff is indeed much more weaker :crazy:
Anyway - i would start with the the usual way of checking for the LSD presence:
Jack up both rear wheels and move one tire by hand - if the tires move in the same direction (or one doesn't move), it means it has LSD.
If the other tire spins in opposite direction - it means the diff is open.