Vibrational state specific dipole moments are diagnostic of the degree of localization of vibrational states in highly vibrationally excited HCN. Using a newly calculated global ab initio dipole moment function and previously calculated highly accurate vibrational wave functions, we show that delocalized (i.e., isomerizing) vibrational states of HCN possess markedly lower dipole moments than localized HCN or HNC states. We also show that the vibrational quantum number dependence of the dipole moment can be used to distinguish delocalized states from localized Franck-Condon-dark states that are made observable by perturbations with localized Franck-Condon-bright states. Furthermore, using classical trajectory analysis we introduce and describe a new experimental approach to obtain these data, which relies on combining optical pumping and state specific molecular transport with hexapoles. With this method it is possible to determine state specific dipole moments with high accuracy and precision.