We demonstrate a practical way to identify the presence of a perovskite phase in rare-earth nickelates (RNiO3) using X-ray photoelectron spectroscopy (XPS). By varying the calcination temperature, we prepared RNiO3 powders with different degrees of chemical reaction. We found that perovskite RNiO3 becomes predominant after high-temperature calcination (>= 1,000 degrees C) in X-ray diffraction and XPS (at Ni 3p and O 1s edges) measurements. While the observed spectra at the Ni 3p edge are similar for all powders, a sizable difference was observed in the O 1s-edge spectra depending on the calcination temperature. With the formation of a perovskite phase with a trivalent Ni3+ state, an XPS peak corresponding to oxygen ions in the perovskite lattice distinctly emerges. Our work shows that the Ni3+ state cannot be determined by analyzing the Ni3+ edge solely and rather, the O is edge should be simultaneously monitored for explicit identification.