  The detection of gravitational wave signals by Advanced LIGO and Advanced
Virgo enables us to probe the polarization content of gravitational waves. In
general relativity, only tensor modes are present, while in a variety of
alternative theories one can also have vector or scalar modes. Recently test
were performed which compared Bayesian evidences for the hypotheses that either
purely tensor, purely vector, or purely scalar polarizations were present.
Indeed, with only three detectors in a network and allowing for mixtures of
tensor polarizations and alternative polarization states, it is not possible to
identify precisely which non-standard polarizations might be in the signal and
by what amounts. However, we demonstrate that one can still infer whether, in
addition to tensor polarizations, alternative polarizations are present in the
first place, irrespective of the detailed polarization content. We develop two
methods to do this for sources with electromagnetic counterparts, both based on
the so-called null stream. Apart from being able to detect mixtures of tensor
and alternative polarizations, these have the added advantage that no waveform
models are needed, and signals from any kind of transient source with known sky
position can be used. Both formalisms allow us to combine information from
multiple sources so as to arrive at increasingly more stringent bounds. For now
we apply these on the binary neutron star signal GW170817, showing consistency
with the tensor-only hypothesis with p-values of 0.315 and 0.790 for the two
methods.
