How to tell if flywheel is iron or steel

Well to answer the poster's question, an iron FW will typically not be fully machined all over and show some rough cast surfaces on one or both sides. In addition, if there are raised cast numbers or letters, it's iron. Steel FWs are fully machined all over, have no rough pebble like cast surfaces, and if there are any numbers or words they are stamped in.
That's what I was gonna say. If it has texture like a cast iron head or block, likely cast. Smooth on the non clutch side, probably steel.