Energy
Hi,
Any clumping pretty much says it is KNO[SUB]3[/SUB].

I think the link above mentions that along with a slight “moldy” odor, a taste will leave a “moldy” aftertaste.

KNO[SUB]3[/SUB] will be a dingy yellow next to KH[SUB]2[/SUB]PO[SUB]4[/SUB].
KH[SUB]2[/SUB]PO[SUB]4[/SUB] will be bright white free flowing, no clumping.
KH[SUB]2[/SUB]PO[SUB]4[/SUB] dissolves more readily. In a few milliliters of distilled water a gram of KH[SUB]2[/SUB]PO[SUB]4[/SUB] will dissolve readily, a gram of KNO[SUB]3[/SUB] will take a lot longer.
Finally and maybe more fun mix equal parts of the unknown substance with sugar and light with a torch, the KNO[SUB]3[/SUB] will be energetic.
Hope this helps,

Biollante