This is the puzzle I did today -- the "cut" is 48-piece classic. I typically do the 100-piece classic. If you want to do the puzzle, just click on the arrow and it will take you to jigzone.com -- you can change the cut yourself.
When you get there, the pieces will be all mixed up. When you've joined the right piece, you'll feel a little click and you won't be able to separate the pieces. That's how you know you got the right one.
Note it also times you, and I get very competitive about this. The more I do, the more often I can beat the average of other players.