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 model within the mean-field approximation. The model is formulated at finite temperature and for non-vanishing net-quark and isospin chemical potentials. The fermion interactions are controlled by the strength of the scalar and vector couplings in the isoscalar and isovector channels of constituent quarks. We explore properties and differences in the behaviour of the net-quark number and isovector susceptibilities for different values of thermal parameters near the phase transition. We argue that any non-monotonic behaviour 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.