By performing high-resolution electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) experiments on nitrogen atoms encapsulated in C-60 the capability of the quartet spin system to sense small local fields at the site of the atom is demonstrated. Symmetry lowering induced by a phase transition in polycrystalline C-60 at 258 K can easily be detected by the appearance of a zero-field splitting of axial symmetry. Freezing of cage rotation is observed via the magnetic dipole interaction with C-13 nuclei of the carbon shell.