Which iron sulfide are you talking about? There are several. FeS2 looks like gold, and would not be mistaken for iron. An easy test would be to see if a magnet attracts the material, or if electricity can be conducted through it. If it does, it it metallic iron.
You can add hydrochloic acid and notice if the smell of H2S is detected.
See also
http://www.answers.com/topic/iron-ii-sulfide
a chemical test to distinguish between a sample of iron and a sample of iron sulphide.
2 answers
I answered this one recently.