This would be very easy to verify though because the original punches have small sections that are not cut, so the punch is contiguous with the rest of the cardback paper (front and back). Jamming a different punch in probably wouldn't line up, but even if it did the material would be broken there.
I don't know, I've replaced punchtabs before when they haven't been removed completely from the card and it's almost impossible to tell the difference. Either way, it's something so subtle that most collectors are probably not going to pick up on it.
That's the primary reason that I never pay extra for a figure just because it's listed as "unpunched."