  The synchrotron radiation from secondary electrons and positrons (SEPs)
generated by hadronic interactions in the shock of supernova remnant (SNR)
could be a distinct evidence of cosmic ray (CR) production in SNR shocks. Here
we provide a method where the observed gamma-ray flux from SNRs, created by
pion decays, is directly used to derive the SEP distribution and hence the
synchrotron spectrum. We apply the method to three gamma-ray bright SNRs. In
the young SNR RX J1713.7-3946, if the observed GeV-TeV gamma-rays are of
hadronic origin and the magnetic field in the SNR shock is $B\gtrsim 0.5$mG,
the SEPs may produce a spectral bump at $10^{-5}-10^{-2}$eV, exceeding the
predicted synchrotron component of the leptonic model, and a soft spectral tail
at $\gtrsim 100$keV, distinct from the hard spectral slope in the leptonic
model. In the middle-aged SNRs IC443 and W44, if the observed gamma-rays are of
hadronic origin, the SEP synchrotron radiation with $B\sim 400 - 500 \mu$G can
well account for the observed radio flux and spectral slopes, supporting the
hadronic origin of gamma-rays. Future microwave to far-infrared and hard X-ray
(>100keV) observations are encouraged to constraining the SEP radiation and the
gamma-ray origin in SNRs.
