It couldn't be easier. A magnet would be attracted to a steel disc. 
Steel discs are definitely heavier than aluminum discs, but except for
rarely found thick professional grade are not really sturdier than
aluminum. They tend to permanently bend easier, especially in the more
common home grade like Wilcox-Gay Recordio and dozens of other brands. 
One other problem is rust. The metal can rust under the lacquer coating
and create bubbles and loosen adhesion of the lacquer layer.

Also note if the magnetic pickup you would be using would likewise be
magnetically attracted to the disc which would raise the tracking

Mike Biel [log in to unmask] 

-------- Original Message --------

From: John Dawson <[log in to unmask]>
> Hi everyone,
> I was wondering if anyone had tips/resources about determining whether or not a the substrate of a 
> laquer is steel or aluminum. I have some WWII era discs and some are definitely heavier and sturdier > than others. I am wondering if these are steel. 
> John Dawson Indiana University Media Preservation Initiative