Cold-atom-based ion sources (CABIS) are a timely example of how laser cooling and trapping can be used in practical applications, such as nanofabrication and microscopy. Unfortunately, Coulomb repulsion between ions can significantly deteriorate CABIS performance. The authors demonstrate that embedded Rydberg atoms can be used for noninvasive, near-real-time measurement of the electric fields between ions in CABIS, and thereby to control Coulomb-induced degradation via feedback. The spectra of different Rydberg states reveal which states are suitable for low- and high-field monitoring, and this work also sheds light on many-body interactions between ultracold ions and neutral atoms.