  Indirect detection of dark matter is a major avenue for discovery. However,
baryonic backgrounds are diverse enough to mimic many possible signatures of
dark matter. In this work, we study the newly proposed technique of dark matter
velocity spectroscopy\,\cite{speckhard2016}. The non-rotating dark matter halo
and the Solar motion produce a distinct longitudinal dependence of the signal
which is opposite in direction to that produced by baryons. Using collisionless
dark matter only simulations of Milky Way like halos, we show that this new
signature is robust and holds great promise. We develop mock observations by
high energy resolution X-ray spectrometer on a sounding rocket, the Micro-X
experiment, to our test case, the 3.5 keV line. We show that by using six
different pointings, Micro-X can exclude a constant line energy over various
longitudes at $\geq$ 3$\sigma$. The halo triaxiality is an important effect and
it will typically reduce the significance of this signal. We emphasize that
this new {\it smoking gun in motion} signature of dark matter is general, and
is applicable to any dark matter candidate which produces a sharp photon
feature in annihilation or decay.
