Ta-da! Whenever she finishes a puzzle, she either shouts, "Ta-da" or has to give someone a high five!
Since this was getting pretty easy for her, a few months ago we moved her up to the 24-30 piece range, and now she's working on the 48 piece puzzles. These are a challenge for her, but with a little help and encouragement she's able to figure it out!
The other thing she likes to do when she's finished with a puzzle, is run her hands over the top to "smooth it out." I guess she's seen me do that a couple times and has picked up on that! :-)