  We assess the ability of the Belle II experiment to probe the Dirac or
Majorana nature of a massive right-handed neutrino (RHN) $N$ in the MeV to GeV
mass range. We consider the production and decay of RHNs to proceed via new
interactions described by the standard model effective field theory (SMEFT)
extended with right-handed neutrino fields (SMNEFT), and not via mass mixing
with active neutrinos. We find that Belle II has the potential to discover $N$
if kinematically accessible. We perform detailed simulations of the angular
distributions of lepton pairs from the decay of $N$ produced in two-body and
three-body decays of $B$ mesons. We show that for $m_N$ above 100 MeV, Belle II
can distinguish between Dirac and Majorana neutrinos at more than the 5$\sigma$
CL for most operators, and the combination of the production and decay
operators can be identified from the subsequent decay of the heavy neutrino.
Also, the production operators can be identified using three-body $B$ meson
decay for any $m_N$ if the $B\to D\ell N$ and $B\to D^*\ell N$ events can be
well separated.
