We mark them with a synthetic DNA strand that can only be seen with a special laser frequency, calibrated to that exact DNA sequence.
It is an extremely effective way to combat potential fakes because the DNA link has an astronomical 1-in-33 trillion chance of being accurately reproduced.
The ball can change hands a thousand-plus times, but it will never lose that DNA. The chance of replicating this exact DNA sequence is one in 33 trillion, so it's virtually impossible.