  Autonomous radio-detection, i.e., detection of air-showers with standalone
radio arrays, is one of the major technical challenges to overcome for the next
generation astroparticle detectors. In this context, we study polarisation
signatures of simulated radio signals to perform an identification of the
associated air-showers initiated by cosmic-rays and neutrinos. We compare the
two sources of radio emission (the charge excess and geomagnetic) and show that
the former is almost negligible for inclined (zenith angle $>65^{\circ}$)
cosmic-ray air-showers. This provides an efficient background rejection
criterion at the DAQ level, based on the projection of the total electric field
along the direction of the local magnetic field. This relevant quantity can be
easily computed -- even in an online treatment -- for antennas measuring three
orthogonal polarisations. Independently of the experimental antenna layout, we
estimate that assuming a random polarisation of noise events, a rejection from
$\approx 72\%$ (for a non favorable detector location) to $\approx 93\%$ (for a
favorable location) of the noise induced events and a trigger efficiency of 87%
(93%) with a $3\sigma$ ($5\sigma$) trigger threshold level should be
achievable. We also show that neutrino-induced showers present a charge excess
to geomagnetic signal ratio up to $\sim 10$ times higher than for cosmic ray
showers. Although this characteristic makes the identification of
neutrino-induced showers challenging via the method developed here, it provides
an efficient criterion to perform an offline discrimination between cosmic-ray
and neutrino primaries. The stronger charge excess emission will also help the
reconstruction of air-shower parameters, such as the core position.
