  The first direct detections of gravitational waves from merging binary black
holes open a unique window into the binary black hole formation environment.
One promising environmental signature is the angular distribution of the black
hole spins; systems formed through dynamical interactions among already-compact
objects are expected to have isotropic spin orientations whereas binaries
formed from pairs of stars born together are more likely to have spins
preferentially aligned with the binary orbital angular momentum. We consider
existing gravitational wave measurements of the binary effective spin, the
best-measured combination of spin parameters, in the four likely binary black
hole detections GW150914, LVT151012, GW151226, and GW170104. If binary black
hole spin magnitudes extend to high values we show that the data exhibit a
$2.4\sigma$ ($0.015$ odds ratio) preference for an isotropic angular
distribution over an aligned one. By considering the effect of 10 additional
detections, we show that such an augmented data set would enable in most cases
a preference stronger than $5\sigma$ ($2.9 \times 10^{-7}$ odds ratio). The
existing preference for either an isotropic spin distribution or low spin
magnitudes for the observed systems will be confirmed (or overturned)
confidently in the near future.
