  Magnetic fields are crucial in numerous astrophysical processes within the
interstellar medium. However, the detailed determination of magnetic field
geometry is notoriously challenging. Based on the modern magnetohydrodynamic
(MHD) turbulence theory, we introduce a novel statistical technique, the
"Y-parameter", to decipher the magnetic field inclination in the ISM and
identify dominant turbulence modes. The Y-parameter, calculated as the ratio of
anisotropies of different Stokes parameter combinations, displays contrasting
trends with the mean-field inclination angle in Alfv\'enic and compressible
turbulence modes. A Y-parameter value around $1.5\pm0.5$ provide a statistical
boundary to determine the dominant MHD turbulence modes. We have discovered
specific correlations between the Y-parameter value and the inclination angle
that unveil the dominant turbulence mode. This methodology, when applied to
future radio polarisation surveys such as LOFAR and SKA, promises to
significantly enhance our knowledge of 3D magnetic field in the ISM and improve
our understanding of interstellar turbulence.
