  We explore discrimination of two types of leptonic Yukawa interactions
associated with Higgs triplet, $\bar L^c_L \Delta L_L$, and with $SU(2)$
singlet doubly charged scalar, $\bar e_R^c k^{++} e_R$. These interactions can
be distinguished by measuring the effects of doubly charged scalar boson
exchange in the $e^+ e^- \to \ell^+ ell^-$ processes at polarized
electron-positron colliders. We study a forward-backward asymmetry of
scattering angular distribution to estimate the sensitivity for these effects
at the ILC. In addition, we investigate prospects of upper bounds on the Yukawa
couplings by combining the constraints of lepton flavor violation processes and
the $e^+ e^- \to \ell^+ \ell^-$ processes at the LEP and the ILC.
