The cosmic ray spectrum has been shown to extend well beyond 10^20 eV. With nearly 20 events observed in the last 40 years, it is now established that particles are accelerated or produced in the universe with energy near 10^21 eV. In all production models neutrinos and photons are part of the cosmic ray flux. In acceleration models (bottom-up models), they are produced as secondaries of the possible interactions of the accelerated charged particle, in direct production models (top-down models) they are a dominant fraction of the decay chain. In addition, hadrons above the GZK threshold energy will also produce, along their path in the Universe, neutrinos and photons as secondaries of the pion photo-production processes. Therefore, photons and in particular neutrinos, are very distinctive signatures of the nature and distribution of the potential sources of ultra high energy cosmic rays. In the following we expose the identification capabilities of the Auger observatories. In the hypothesis of nu_mu-->nu_tau oscillations with full mixing, special emphasis is placed on the observation of tau neutrinos, with which Auger is able to establish the GZK cutoff as well as to provide a strong and model independant constraint on the top-down sources of ultra high energy cosmic rays.