  Nonlinear triadic interactions are at the heart of our understanding of
turbulence. In flows where waves are present modes must not only be in a triad
to interact, but their frequencies must also satisfy an extra condition: the
interactions that dominate the energy transfer are expected to be resonant. We
derive equations that allow direct measurement of the actual degree of
resonance of each triad in a turbulent flow. We then apply the method to the
case of rotating turbulence, where eddies coexist with inertial waves. We show
that for a range of wave numbers, resonant and near-resonant triads are
dominant, the latter allowing a transfer of net energy towards two-dimensional
modes that would be inaccessible otherwise. The results are in good agreement
with approximations often done in theories of rotating turbulence, and with the
mechanism of parametric instability proposed to explain the development of
anisotropy in such flows. We also observe that, at least for the moderate
Rossby numbers studied here, marginally near-resonant and non-resonant triads
play a non-negligible role in the coupling of modes.
