As a metallurgist and a car guy, you asked a really tough question to explain.
It's always a good idea to touch it with a magnet, that can rule out aluminum and many stainless steels if it isn't magnetic.
As was already mentioned, you can tell the difference in the way the parts are formed. A stamped part on these trucks will most likely (although not always) be stainless.
As far as pitting, Aluminum is very resistant to corrosion damage, so if the part has corrosion or pitting it's more likely it's stainless steel or pot metal.
