How to distinguish the metamorphosed of ferrous sulfate?

Ferrous sulfate in air are easy to be oxidized to ferric sulfate containing ferric iron, not easy to be absorbed by crops and metamorphism at this time. How to distinguish the metamorphosed of ferrous sulfate?
 
The simplest method is to use potassium thiocyanate reagent as identification to indentify the ferrous sulfate. Specific method is to place the ferrous sulfate solution in a beaker, and then slowly add potassium thiocyanate solution. If the solution turned red, the ferrous sulfate is bad. The reason is very simple, potassium cyanide this reagent Fe3 + will become blood red, form a kind of difficult to decompose complex.

With potassium thiocyanate identification of ferrous sulfate is metamorphic method is simple and high accuracy. If you are buying the ferrous sulfate hepotahydrate and ferrous sulfate monohydrate have questions whether metamorphism, can use this method to identify. The shelf life of ferrous sulfate is about half a year, generally kept improper methods will cause product deterioration, so need to seal and storage after purchase