Weight is a good indicator of the type of metal. Iron, steel, stainless steel and aluminum are all heavier than plastic.
Appearance can also help determine the material. Iron has a rust color, steel does not, stainless steel tends to be shiny, and aluminum is light in color.
Magnetism is another indicator. Iron and steel are magnets (they have a north pole), while stainless steel and aluminum are not as strong (have a south pole).