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Some other answers (that kind of fix the issue that you might not be showing the exact Waldo):

Make a copy of the page and have your friend draw a unique repeating pattern on the entire back. Cut out Waldo from the page and have your friend verify the design “signature” on the back.

For a better design signature have your friend paint the back of the copy with a color made from three unique rgb values they choose that you don’t know. When you cut Waldo out of the copy have them verify the color matches their unique color.

Another qualification is that details about Waldo are public so simply describing the stripes on his shirt and such doesn’t help prove to your friend that you’ve found him.