FULL DISCLOSURE: I really enjoyed performing this! I always got a good reaction and was never "caught." That said, the symbols on the back of these wooden discs are marked. The markings are slight. However, because they are made of wood, the wood grain pattern is different on each as well. I always found it easier to just look at the wood grain instead of the markings. It does not take away from the fact that the spectator gets to choose the discs and where they go (sort of). It is still a great effect!
From the description:
One of the most influential pieces of mentalism in the modern age! Your audience’s illusion of choice will be under your complete control when you perform Free Will by Deddy Cobuzier.
Here’s what happens: