Hope I didn't confuse things too much. I may know a little bit about metal, but don't know much about mess tins :?:
Assuming it doesn't respond to a magnet, the only way I know to tell the difference between steel and polished aluminium is by feel and sight. Aluminium is only about one third the density of steel and therefore feels one third of the weight for any given volume, but it's not as strong so tends to be made a bit thicker.
It's still worth doing the magnet test, but it's only conclusive if the metal responds.
It's not as much help as I'd like to give but it's the best I can do on the internet :biggthump