2 different ways that I am aware of to tell if SWIY got burned from some scandalous street tweeker.
You can prefire with I2 to see if it reacts or you can take a sample of your "supposed" hypo, add a touch of sulfuric acid to it. SO2 (sulfur dioxide gas should bee released) and elemental sulfur should drop to the bottom.
At least that is what Geez said when he schooled Swim.