To tell whether you have a pot-type or flat type, unbolt the little "window" from the bottom of the clutch bellhousing, and you can see the flywheel. If you look at the picture posted above, that's a pot one. The flat one is much thinner and, well, flat.
In addition, on a Calibra, if you have a pot type flywheel, you can't separate the gearbox and engine whilst the engine is in the car, you have to remove the lot from underneath the car by unbolting the front subframe. Pain.
I believe ALL distributorless M2.8 XEs had pot-flywheels, and all distributored models have flat type flywheels.
Everyone else, if this mod has no major negative effects, why is the flywheel so heavy and made that shape in the first place? Surely vauxhall didn't spend millions developing a flywheel which is four and a half pounds too heavy?