The rotational band structure of a magnetic dipole vibronic transition is exactly the same as that of an electric dipole vibronic transition. It is often assumed that molecular vibronic transitions are electric dipole, introducing an ambiguity about the change in symmetry if the transition is weak enough to be an allowed magnetic dipole transition and the symmetry of either vibronic state is unknown. In atomic spectroscopy, it is known that either the Zeeman effect or the polarization dependence of wide angle fluorescence interference can be used to distinguish among electric-dipole, magnetic-dipole, electric-quadrupole, enforced-dipole transitions, etc. This note points out that rotationally resolved double resonance using polarized light can be used to determine the type of an unknown multipole transition if the type (magnetic or electric) of the other double resonance transition is known. Stark and Zeeman effects are valuable special cases since the type of the Stark (or Zeeman) transition moment is known a priori to be electric (or magnetic) dipole. Published Stark spectra of formaldehyde-d2 experimentally prove Innes’ assignment of a magnetic dipole transition.