Give a fine powdered mixture of your substance and $\ce{CaF2}$ a lead crucible. Add $1~\pu{ml}$ of concentrated sulfuric acid and warm gently.
If your substance is $\ce{SiO2}$ then gaseous $\ce{SiF4}$ will form:
$$\ce{2 F- + H2SO4 -> 2 HF + SO4^{2-}} \tag{1}$$ $$\ce{SiO2 + 4 HF -> SiF4 + 2 H2O} \tag{2}$$
A moistened black filter paper placed on the cap of the crucible will show white $\ce{SiO2}$ when dried (reaction (2) reverted).
As an alternative to the black filter paper you could place a water drop hanging at a black glass rod above the small hole in the crucible's cap. You will see silicic acid at the drop if $\ce{SiO2}$ is present.