How do we know the charges of quarks?
  Experimental tests performed in the past to determine quark charges are reviewed. It is pointed out that only experiments involving two real photons can really distinguish between the Gell-Mann-Zweig scheme of fractional charges from the Han-Nambu scheme of integer charges in the context of a theory with gauged colour interactions. While many experiments with two real photons have been performed, the only experiment with a definitive statement on quark charges was one on hard scattering of real photons at CERN. Some theoretical issues not fully resolved are pointed out. 
Saurabh D.Rindani