  The $\mu^+ \to e^+ X$ decay, where $X$ is a dark sector boson, provides one
of the strongest available bounds on the scale of dark sector interactions. The
$X$ boson can be an axion or a dark photon. We show that the concurrent
determination of the anti-muon and positron polarizations makes possible to
distinguish with a confidence level of 99% between the two dark sector portals
with as few as 6 observed events in the case of the massless dark photon.
Instead, the massive spin-1, dimension 4 dark portal cannot be distinguished
from the axion-like case. We also discuss the possibility that the $X$ boson be
a massive spin-2 particle.
