  In this paper we propose a novel technique to constrain the progenitor binary
black hole (BBH) formation history using the remnant masses and spins of merged
black holes (BHs). Exploring different models, we found that dynamically formed
BBHs are distributed differently in the mass-spin plane than those that have
formed in isolation. Stellar evolution recipes crucially affect the remnant
mass distribution, suggesting that future efforts should be devoted to finding
a common way of modelling the evolutionary phases of single and binary stars.
Our simple approach has allowed us to place weak constraints on the origin of
the presently observed population of merged BHs, although with high
uncertainties. Our results show that the fingerprints of different BBH
formation channels will emerge as soon as LIGO detects more than $\sim 10^2$
merger events. This work provides a way of thinking that can be easily used
both by people working on isolated and on dynamical BBH formation and
evolution.
