We demonstrated the accurate prediction of a quasibound spectrum of a negative ion using a high-precision theoretical approach. We used La as a test case due to a recent experiment that measured energies of 11 resonances in its photodetachment spectrum attributed to transitions to quasibound states [Phys. Rev. A 102, 042812 (2020)]. We identified all of the observed resonances and predicted one more peak just outside the range of the prior experiment. Following the theoretical prediction, the peak was observed at the predicted wavelength, validating the identification. The same approach is applicable to a wide range of negative ions.