  Semiconductors in the proximity of superconductors have been proposed to
support phases hosting Majorana bound states. When the systems undergo a
topological phase transition towards the Majorana phase, the spectral gap
closes, then reopens, and the quasiparticle band spin polarization is inverted.
We focus on two paradigmatic semiconductor-superconductor heterostructures and
propose an all-electrical spectroscopic probe sensitive to the spin inversion
at the topological transition. Our proposal relies on the indirect coupling of
a time-dependent electric field to the electronic spin due to the strong Rashba
spin-orbit coupling in the semiconductor. We analyze within linear response
theory the dynamical correlation functions and demonstrate that some components
of the susceptibility can be used to detect the nontrivial topological phases.
