Identify the oxygen atoms that undergo reduction in the reaction below:
$$\ce{O2 + Cr^3+ -> H2O2 + Cr2O7^2-}$$
I know that the oxygen atoms in $\ce{O2}$ have an oxidation state of $0$.
In the product $\ce{H2O2}$, the oxygen atoms have an oxidation state of $-1$; and in $\ce{Cr2O7^2-}$, oxygen atoms have an oxidation state of $-2$.
Based on how the problem was solved on the answer key it was assumed that all the oxygen atoms from $\ce{O2}$ are reduced to peroxide atoms.
How do we know that there are no oxygen atoms that are oxidized to become chromate oxygens with a $-2$ oxidation state?
I understand that there are other $\ce{H2O}$ oxygen atoms available to provide the chromate oxygens that would change the oxidation state.