  We probe the effective field theory extending the Standard Model with a
sterile neutrino in B meson decays at B factories and lepton colliders, using
angular and polarization observables. We put bounds on different effective
operators characterized by their distinct Dirac-Lorentz structure, and probe
the $N$-mediated B decays sensitivity to these interactions. We define a
Forward-Backward asymmetry $A_{FB}^{\ell \gamma}$ between the muon and photon
directions for the $B \to \mu \nu \gamma$ decay, which allows us to separate
the SM contribution from the effective lepton number conserving and violating
processes, mediated by a near on-shell $N$. Using the most stringent
constraints on the effective parameter space from Belle and BaBar we find that
a measurement of the final polarization $P_{\tau}$ in the rare $B^-\rightarrow
\ell^-_{1}\ell^-_{2} \pi^+$ decays can help us infer the scalar or vector
interaction content in the $N$ production or decay vertices. We find that the B
meson decays are more sensitive to scalar operators.
