  We present results from our analysis of the finite-temperature properties of
the spin 1/2 $J_{1}$-$J_{2}$ Heisenberg model on a square lattice. The analysis
is based on the exact diagonalization of small clusters with 16 and 20 sites
utilizing the finite-temperature Lanczos method. In particular, we focus on the
temperature dependence of the third-order magnetic susceptibility as a method
to resolve the ambiguity of exchange constants. We discuss the entire range of
the frustration angle $\phi=\tan^{-1}(J_{2}/J_{1})$ parameterizing the
different possible phases of the model, including the large region in the phase
diagram with at least one ferromagnetic exchange constant.
