  We investigate how to use information on the effective spin parameter of
binary black hole mergers from the LIGO-Virgo gravitational wave detections to
discriminate the origin of the merging black holes. We calculate the expected
probability distribution function for the effective spin parameter for
primordial black holes. Using LIGO-Virgo observations, we then calculate odds
ratios for different models for the distribution of black holes' spin magnitude
and alignment. We evaluate the posterior probability density for a possible
mixture of astrophysical and primordial black holes as emerging from current
data, and calculate the number of future merger events needed to discriminate
different spin and alignment models at a given level of statistical
significance.
