Presence of peroxide in ethers of old stock can be tested by first treating them with $\mathrm{F}\mathrm{e}\mathrm{S}{\mathrm{O}}_4$ solution and then adding an aqueous solution of KCNS.
Ethers form peroxides, when come in contact with air and light.

$\mathrm{F}{\mathrm{e}}^{3+}+\mathrm{K}\mathrm{C}\mathrm{N}\mathrm{S}\to$ Red colour