Topps Vault has sometimes offered cards with blank backs labeled as proof cards. And some blank cards, like the 67 Maris as a Yankee, the 77 Robinson as an Oriole, or the the Topps 1984 Encased or head fully in the box cards, would all seem to be legitimate proofs since they differ from the finally issued cards. Sometimes proof cards are also unfinished or missing some ink.
But I think without some direct provenance or tell tale markings it would be virtually impossible to distinguish a true proof from a blank back misprint. Perhaps others will have better insights.
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