I have a pair of 15x7 strait spoke wheels that sort of look like magnesium. They have zero markings of any kind on them. How can I tell if they are magnesium? They are mounted on tires and both tires were flat. I know magnesium will bleed air, so I aired them up and will see if both eventually go flat. I also filed a small area on the back and put a lighter to the shavings. They sparked slightly. Is there any other method on how to tell if these are magnesium or aluminum?