One easy way to tell the difference between aluminum and stainless steel is to simply look at it.
Stainless steel has a shiny look to it while aluminum will have a dull look to it.
You can also check for stainless steel vs aluminum with a grinder but only use the grinder test if the item is scrap metal.
If you grind the metal and it throws sparks then it's stainless steel but if you grind the metal and there is no sparks coming from it then the item is aluminum.