A Lightened Flywheel will have a fairly large amount of material removed from the back, but if you have not got a standard flywheel to compare to, then it may be hard to spot. A Lightened non-EFi flywheel ought to weigh something like 5kg I would have thought, so there's another way.

Alternatively, post a pic of the back of it (not the friction face).

How much is it anyway? As I can get them done fairly cheaply.