The recently launched satellite, Fermi Gamma-ray Space Telescope , is expected to find out if cosmic-ray (CR) protons are generated from supernova remnants (SNRs), especially RX J1713.7-3946, by observing the GeV-to-TeV γ-rays. The GeV emission is thought to be bright if the TeV emission is hadronic, i.e., of proton origin, while dim if leptonic. We reexamine the above view using a simple theoretical model of nonlinear acceleration of particles to calculate the gamma-ray spectrum of Galactic young SNRs. If the nonlinear effects of CR acceleration are considered, it may be impossible to distinguish the evidence of proton acceleration from leptonic in the γ-ray spectrum of Galactic young SNRs like RX J1713.7-3946. On the other hand, future km3-class neutrino observations will likely find a clear evidence of the proton acceleration there.