Magnetotransport at low temperatures and high-magnetic fields has been used to probe the spin splitting of the electron and hole Landau bands in natural graphite. Tilting the sample in the magnetic field allows to tune the Zeeman energy and hence the spin splitting. Due to the movement of the Fermi energy in the magnetic field, it is necessary to use the full Slonczewski, Weiss, and McClure Hamiltonian to extract the spin splitting. The effective $g$ factor is found to be ${g}_{s}=2.5\ifmmode\pm\else\textpm\fi{}0.1$ with no measurable anisotropy within experimental error. This value is somewhat larger than the value of the anisotropic $g$-factor determined using electron spin resonance, which is attributed to the many body contribution to the transport $g$ factor.