Simon's reagent is used as a simple spot-test to presumptively identify alkaloids as well as other compounds.
It reacts with secondary amines like MDMA and methamphetamine to give a blue solution.
When exposed to an amine, reaction with acetaldehyde produces the enamine, which subsequently reacts with sodium nitroprusside to the imine.
Finally, the iminium salt is hydrolysed to the bright blue[1] Simon-Awe complex.
[3][5] Acetaldehyde can be replaced with acetone, in which case the reagent detects primary amines instead, giving a purple coloured product.