  The photon polarization in $D_{(s)} \to K_1 (\to K\pi\pi) \gamma$ decays can
be extracted from an up-down asymmetry in the $K \pi \pi$ system, along the
lines of the method known to $B \to K_1 (\to K\pi\pi) \gamma$ decays. Charm
physics is advantageous as partner decays exist: $D^+ \to K_1^+ (\to K\pi\pi)
\gamma$, which is standard model-like, and $D_s \to K_1^+ (\to K\pi\pi)
\gamma$, which is sensitive to physics beyond the standard model in $|\Delta c|
=|\Delta u|=1$ transitions. The standard model predicts their photon
polarizations to be equal up to U-spin breaking corrections, while new physics
in the dipole operators can split them apart at order one level. We estimate
the proportionality factor in the asymmetry multiplying the polarization
parameter from axial vectors $K_1(1270)$ and $K_1(1400)$ to be sizable, up to
the few ${\cal{O}}(10)\%$ range. The actual value of the hadronic factor
matters for the experimental sensitivity, but is not needed as an input to
perform the null test.
