  We report a magnetic field dependence of fluorescence of diamond single
crystals containing NV$^-$ centers. In such spectra, numerous sharp lines are
found, which correspond to Level Anti-Crossings (LACs) in coupled spins systems
comprising an NV$^-$ center. Theoretical modeling of such "LAC-spectra" enables
characterization of paramagnetic defect centers and determination of their
magnetic resonance parameters, such as zero-field splitting and hyperfine
coupling constants. The outlined method thus enables sensitive detection of
paramagnetic impurities in diamond crystals.

Mesoscale and Nanoscale Physics;Chemical Physics