We present a new diagnostic for open-shell coupled-cluster theory, readily computed from the single substitution amplitudes in the CCSD wavefunction. The new diagnostic, D-1(ROCCSD), is designed to be comparable to the previously proposed D-1(CCSD) diagnostic. Unlike other approaches, the D-1 diagnostics are independent of system size and have the same invariance properties as the energy with respect to orbital rotations. Calibration of the D-1(ROCCSD) diagnostic on 34 molecular systems indicates that for Values of D-1(ROCCSD) of 0.025 or below the quality of the CCSD results are, in general, excellent, whereas values larger than 0.025 signal inadequacies in the CCSD approach.