  In view of the proliferation in the number of new charmonium states, it is
really important to have a experimental way to prove that an observed bump is,
indeed, a real resonance. To do that, in this paper we present an alternative
method to demonstrate the resonant behavior of a state. With this method, the
phase variation of a generic complex amplitude can be directly revealed through
interference in the Dalitz-plot region where it crosses a well established
resonant state, used as a probe. We have tested the method for the $Z^-(4430)$
state by generating Monte Carlo samples for the $B^0\to \psi^\prime \pi^-K^+$
decay channel. We have shown that the proposed method gives a clear oscillation
behavior, related to the phase variation associated to a real resonant state,
in the case where the $Z^-(4430)$ is considered as a regular resonance with a
strong phase variation. We have also discussed the possibility of using the
proposed method complementary to the Argand diagram to determine the internal
structure of the $Z^-(4430)$ state.
