  The magnetic susceptibility $\chi(T)$ of spin-1/2 chains is widely used to
quantify exchange interactions, even though $\chi(T)$ is similar for different
combinations of ferromagnetic $J_1$ between first neighbors and
antiferromagnetic $J_2$ between second neighbors. We point out that the spin
specific heat $C(T)$ directly determines the ratio $\alpha = J_2/|J_1|$ of
competing interactions. The $J_1-J_2$ model is used to fit the isothermal
magnetization $M(T,H)$ and $C(T,H)$ of spin-1/2 Cu(II) chains in LiCuSbO$_4$.
By fixing $\alpha$, $C(T)$ resolves the offsetting $J_1$, $\alpha$ combinations
obtained from $M(T,H)$ in cuprates with frustrated spin chains.
