Comment by lordnacho
This reminds me of the "Where's Waldo (Wally in UK)" example:
You can prove that you found Wally with a large piece of paper with a hole in it. You move the hole over Wally, and the person you're sitting with can see you found it, but he's no wiser about where.
Another way is to get them to put marks/signatures over the back of the blank. Overlay to e blank, and cut Wally out of it where he occurs on the actual page and give them the cutout.