A couple of things come to mind.
First, other than Campagnolo or Cromodora most "mag" wheel companies sell a lot more Aluminum wheels than Magnesium so they tend to be pretty rare. Actually, very few companies make Magnesium wheels at all.
You mentioned that the centers are cast and the rims polished. Are they three-piece? One-piece wheels of magnesium wouldn't have polished rims (from the factory anyway) because magnesium will corrode quickly.
Do they have Minilite makings cast in? There are a lot of copies of the famous Minilite 8-spoke pattern out there but if Minilite made them they'll say so. Many of the copies are excellent wheels but few of the other manufacturers have any Magnesium products.
Do they have steel inserts or are they all alloy? If they are all alloy you can tell by mass properties. As a rough guess I'd think a 13x6 in Aluminum should weight around 12 to 15 lbs. In Magnesium they would likely be under 10 lbs. If you want to be more precise you can ask Archimedes. OK, so he's been dead for a couple thousand years, he did tell us how to do it.
Dunk a wheel under water and see how much volume it displaces. Divide the weight of the wheel by the volume of water displaced and you find the density of the material. For every liter (1.06qt) of volume water weighs 1 kg (2.2lbs), Magnesium weighs 1.7 kg (3.7 lbs) and Aluminum weighs 2.7 kg (5.9 lbs).
PC.