You can't prove that it was valid. Now, if said priest offered a Catholic Mass, then proceeded to take the Host for Satanic purposes, that would be one thing. But if a consecration were made in the context of a Satanic ritual, that would not be valid.
That makes sense, otherwise one could argue a priest would confer the sacrament by merely performing, "This is My Body", without the rest of the rite.
Take out everything else, would it still be a valid confection of the sacrament?
Matter, bread. Form, the words of consecration, intention, within the confines of a Catholic rite.