In slow collisions of two bare nuclei with the total charge larger than the critical value Zcr ≈ 173, the
initially neutral vacuum can spontaneously decay into the charged vacuum and two positrons. The
detection of the spontaneous emission of positrons would be direct evidence of this fundamental
phenomenon. However, the spontaneously produced particles are indistinguishable from the dynamical
background in the positron spectra. We show that the vacuum decay can nevertheless be observed via
impact-sensitive measurements of pair-production probabilities. The possibility of such an observation is
demonstrated using numerical calculations of pair production in low-energy collisions of heavy nuclei.