For reasons that aren't entirely clear, scientists at the University of Granada have come up with a way of telling the difference between "genuine" and "pirated" CDs:
Through the new technique proposed by the scientists of the Department of Optics of the UGR it is possible to identify if a CD has been recorded using a method or a device different to those used in industrial processes, which allows to differentiate between original CDs and copies. This technique uses the phenomenon of light diffraction on a CD surface to appreciate the differences between original and bootleg CDs, as they generate different types of diffraction models.
Of course, a cheaper way is to compare if you've bought the CD from a shop, or off that bloke Dave who works with Aileen down the council depot. But you don't, presumably, find the same level of research funding for that technique.