  General metric theories in a four-dimensional spacetime allow at most six
polarization states (two spin-0, two spin-1 and two spin-2) of gravitational
waves (GWs). If a sky location of a GW source with the electromagnetic
counterpart satisfies a single equation that we propose in this paper, both the
spin-1 modes and spin-2 ones can be eliminated from a certain combination of
strain outputs at four ground-based GW interferometers (e.g. a network of
aLIGO-Hanford, aLIGO-Livingston, Virgo and KAGRA), where this equation
describes curves on the celestial sphere. This means that, if a GW source is
found in the curve (or its neighborhood practically), a direct test of scalar
(spin-0) modes separately from the other (vector and tensor) modes become
possible in principle. The possibility of such a direct test is thus higher
than an earlier expectation (Hagihara et al. PRD, 100, 064010, 2019), in which
they argued that the vector modes could not be completely eliminated. We
discuss also that adding the planned LIGO-India detector as a fifth detector
will increase the feasibility of scalar polarization tests.
