In my experience proof sheets are normally blank backed, or have only partially finished backs.
If a card has the regularly issued front and back, and is hand cut, it was most likely cut from a finished sheet. For example I have a set of the 74 Deckles on uncut sheets with blank backs. I have some square cut undeckled cards with finished backs and some with partially finished backs, and some with blank backs. I believe the ones with finished backs to have been cut from final sheets that had not been deckled. I do not think because a grading company calls it a proof, it is a proof. I think if they have regular backs they are more likely just unfinished cards.
On the other hand, I doubt if there is a hobby standard definition of a proof card, just as their is no universally accepted definition of a variation, so I guess any unfinished card can be considered a proof card
