  Several celestial bodies in co-orbital configurations exist in the solar
system. However, co-orbital exoplanets have not yet been discovered. This lack
may result from a degeneracy between the signal induced by co-orbital planets
and other orbital configurations. Here we determine a criterion for the
detectability of quasi-circular co-orbital planets and develop a demodulation
method to bring out their signature from the observational data. We show that
the precision required to identify a pair of co-orbital planets depends only on
the libration amplitude and on the planet's mass ratio. We apply our method to
synthetic radial velocity data, and show that for tadpole orbits we are able to
determine the inclination of the system to the line of sight. Our method is
also valid for planets detected through the transit and astrometry techniques.
