How to test for Chromium(III) Oxide
The test for $\ce{Cr(III)}$ can be performed in the solid state using an oxidative melt. You will need


chromium(III) oxide (obviously)
sodium carbonate
potassium nitrate
a Bunsen or Teclu burner
a magnesia trough or a crucible
suitable pliers to hold the trough


Mix the green pigment with up to the threefold amount of sodium carbonate and potassium nitrate, place the mixture on the magnesia trough and melt it.

A yellow melt indicates the formation of sodium chromate according to 

$\ce{Cr2O3 + 3KNO3 + 2Na2CO3 -&gt; 2Na2CrO4 + 3KNO2 + 2CO2 ^}$