  The program {\sc Duo} (Yurchenko {\it et al.}, Computer Phys. Comms., 202
(2016) 262) provides direct solutions of the nuclear motion Schr\"odinger
equation for the (coupled) potential energy curves of open shell diatomic
molecules. Wavefunctions from {\sc Duo} are used to compute Land\'e $g$-factors
valid for weak magnetic fields, the results are compared with the idealized
predictions of both Hund's case (a) and Hund's case (b) coupling schemes. Test
calculations are performed for AlO, NO, CrH and C$_2$. The computed $g_J$'s
both provide a sensitive test of the underlying spectroscopic model used to
represent the system and an indication of whether states of the molecule are
well-represented by the either of the Hund's cases considered. The computation
of Land\'e $g$-factors is implemented as a standard option in the latest
release of {\sc Duo}.
