A simple plan of catching a glimpse of a selected card is to have it inserted at the end and prevent the spectator from pushing it quite home by squeezing the deck. Then, with the card protruding about a quarter of an inch. covertly turn the deck: partially over by passing it to the other hand, and get sight of the index.
Another and better plan is to push the selected card through diagonally, and square up, leaving it protruding at the inner end. In this case the index is at the diagonal corner and more easily seen, and the fact of the card protruding can be covered completely.
Still another plan is to insert the left little finger under the inserted card and slightly tilt up inner left hand corner to note the index.