Question

How would you prove that cuprous oxide is not an element?

Answer 1

By inspection?

Cuprous = copper cation with lower oxidation state, from the Latin, "cuprum"
oxide = `"O"^(2-)` anion

Cuprous oxide uses the old naming scheme, wherein "ous" indicates the lower oxidation state of copper. So, this is `"Cu"_2"O"` (rather than `"CuO"`, cupric oxide).

But even if you didn't know that, that doesn't matter, because we know it contains two different elements. Copper has an atomic number not equal to `8`.

By definition, `"Cu"_2"O"` must therefore be a compound. An element must have a single identity and thus contain atom(s) with one consistent atomic number.

(that's why we specify that `"H"_2`, `"O"_2`, `"F"_2`, `"Br"_2`, `"I"_2`, `"N"_2`, and `"Cl"_2` are "diatomic elements", as opposed to simply "elements".)