Sometimes it's hard to tell. You've got the basics. Extrusion is made by pushing molten aluminum through a form as opposed to pouring it into a cast, so it will be uniform size and shape from one end to the other. It will have "fins" (think window frames and heat sinks). Extrusion will bend. Cast aluminum is usually duller than extrusion or painted and brittle (ie: lawn mower deck). Sheet aluminum is made by pouring molten aluminum into a cast or rolling in a sheet which is then formed into shape (roofing). With practice, you'll get to identifying each by a glance.