Ionization of high-Z atoms has never been experimentally studied at intensities beyond 1021W/cm2 and many fundamental assumptions about this regime are therefore untested; for example, the dominance of sequential tunneling, the importance of collective tunneling, and the impact of manybody correlation effects on the tunneling probability. Here we propose to study the ionization rates of rare gases like argon and xenon at ultra-relativistic laser intensities to answer these open questions. As ionization represents a threshold phenomena, it is ideally suited for the in-situ characterization of a laser focus. Given the predictions in the theoretical literature, the method proposed here will be able to provide a direct and irrefutable proof that a certain intensity threshold has been exceeded. Furthermore, it will allow us to quantify the importance of phase-front aberrations and how they change between low-intensity and high-intensity measurements. A complementary observable are vacuum-accelerated electrons, whose energies carry the signature of the laser intensities explored by the particles. The proposed simultaneous measurement of both ions and electrons will provide significant constraints and allow us to scrutinize our theoretical understanding of strong-field ionization and provide unambiguous measurements of the laser intensity that was actually achieved in a multi-PW experiment.