Distinguishing between SU(5) and flipped SU(5)

We study in detail the d=6 operators for proton decay in the two possible matter unification scenarios based on SU(5) gauge symmetry. We investigate the way to distinguish between these two scenarios. The dependence of the branching ratios for the two body decays on the fermion mixing is presented in both cases. We point out the possibility to make a clear test of flipped SU(5) through the decay channel p \to \pi^+ \bar{\nu}, and the ratio \tau(p \to K^0 e^+_{\alpha}) / \tau(p \to \pi^0 e^+_{\alpha}).