The fate of massive stars and the type of supernova (SN) they produce are closely linked to their final stages of evolution. During these late phases, stars may undergo episodic mass loss, forming circumstellar material (CSM) that can leave observable signatures in the SN spectra, particularly as resonance lines in the near-ultraviolet (NUV). In this talk, I will present a sample of superluminous supernovae (SLSNe), an exceptionally luminous class of SNe, focusing on the NUV spectroscopy to search for signatures of recently ejected CSM shells. I will first discuss two SLSNe in which CSM shells are detected, inferring their properties and the timing of the ejection, along with implications for the underlying mass-loss mechanisms and progenitor systems. I will then extend the analysis to the full sample, modelling the spectral regions where CSM-related features are expected in order to evaluate how common such mass-loss episodes are and to place constraints on undetected CSM shells. These results provide new insights into the final stages of massive star evolution and help constrain the nature of SLSN progenitors.