  A sizable right-handed photon polarization in $b\to s\gamma$ is a clear
signal for new physics. In this work we point out that the photon helicity in
$b\to s\gamma$ can be unambiguously extracted by combining the measurements in
$B\to K_1\gamma$ and the Cabibbo-favored $D\to K_1 e^+\nu$ decay. We propose a
ratio of up-down asymmetries in $D\to K_1 e^+\nu$ to quantify the hadronic
effects. { A method for measuring in experiment the involved partial decay
widths in the ratio is discussed, and experimental facilities like BESIII,
Belle-II and LHCb are likely to measure this ratio. We also give the angular
distribution that is useful to extract the photon polarization in the presence
of different kaon resonances. }
