As you tell the spectator to replace the cut, point to the top of the remaining deck on the table. By casually swiping your thumb across your first finger as you point to the half of the deck on the table, you release some of the salt onto the top card.
Tell the spectator to replace the cut and square the deck so nothing stands out. The spectator has replaced the top half of the deck on top of the salt. The salt effectively marks the spot.
By simply tapping the deck on the side, the deck will break at the spectators cut. You can show the card underneath the top half as being that of the spectators.