[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.

[FridgeSeal]

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.

[fragmede]

https://youtu.be/5qzNe1hk0oY for a video if you can't picture that.