If it was right, I took my blue marker and scribbled off the number of the proof they finished. Then I saw a student look the other blue markers in my supply box with a gleam in his eye. And I realized with this system, there is no way I would be able to keep them from cheating. I changed the scribble to my initials, and then I had a better idea... I dug into the depths of my desk drawer and I got out my single hole puncher and punched a hole over the number when they completed it. The students LOVED it! It's amazing how something so simple totally changes the game. Then they wanted to use the hole puncher - and they would fight over whose turn it was to make the punch. The click of the puncher and the creation of the hole was as satisfying as correctly completing the proof. After I (or they) punched the number, they scrambled up the pieces and took it to the middle table and exchanged it for another proof. They kept doing this until they finished all six proofs.
Typically telling students they would be completing six Geometry proofs to prove lines parallel would be met with moans and groans, but my students were very engaged and motivated to finish the problems. I definitely see my hole puncher making future appearances in class!
Next they are going to try out this Proving Lines Parallel Crossword Puzzle. The outline of the proof is still there, but they have to come up with some missing statements and reasons. They still get the self-checking benefit of the crossword puzzle though.