To a reasonable approximation, the only shredded Parmesan you can buy that is likely to contain anything like Parmesan is the stuff that's shredded on-site by places that also sell wheels/chunks of Parmesan.

OTOH, there's rarely a good reason to buy shredded parmesan even if its real; for most uses you want shredded parmesan, the size/shape of shreds you get from such a product aren't great, and its better (and, in any case, cheaper in the long run) to just get blocks of parmesan and a grater, and in typical use quantities you aren't saving much time or effort by getting pre-grated.