  Neutrinos propagating in dense neutrino media such as those in core-collapse
supernovae can experience fast flavor conversions on scales much shorter than
those expected in vacuum. It is believed that a necessary condition for the
occurrence of fast modes is that the angular distributions of $\nu_e$ and
$\bar\nu_e$ cross each other. However, most of the state-of-the-art supernova
simulations do not provide such detailed angular information and instead,
consider only a few moments of neutrino angular distributions. We here propose
an efficient method to use these available few moments to search for fast modes
in supernova simulations. Our method, which is based on searching for crossings
in the angular distributions, can work with any number of moments provided by
the simulation though a larger number of crossings can be captured when higher
moments are available.
