  We discuss the properties of the net--quark and isovector fluctuations along
the chiral phase transition line in the plane spanned by temperature and baryon
chemical potential. Our results are obtained in terms of the
Nambu--Jona-Lasinio (NJL) model within the mean-field approximation. The model
is formulated at the finite temperature and for non-vanishing net--quark and
the isospin chemical potentials. The fermion interactions are controlled by the
strength of the scalar and vector couplings in the iso-scalar and iso-vector
channels of constituent quarks. We explore properties and differences in the
behavior of the net--quark number and isovector susceptibilities for different
values of thermal parameters near the phase transition. We argue that any
non-monotonic behavior of the net--quark number susceptibility along the phase
transition boundary is an excellent probe of the existence and the position of
the second order endpoint in the QCD phase diagram.
