  We propose a method to determine the phase motion of a complex amplitude in
three body heavy meson decays. We show that the phase variation of a complex
amplitude can be directly revealed through the interference observed in the
Dalitz Plot region where it crosses with a well established resonant state.
This method could be applied to the decays \d3pi and $D^+\to K^-\pi^+\pi^+$, to
determine whether the low mass states $\sigma$ and $\kappa$, suggested by E791,
have phase motions compatible with resonances.
