  A reliable theoretical description of double-beta-decay processes needs a
possibility to test the involved virtual transitions against experimental data.
Unfortunately, only the lowest virtual transition can be probed by the
traditional electron-capture or $\beta^-$-decay experiments. In this article we
propose that calculated amplitudes for many virtual transitions can be probed
by experiments measuring rates of ordinary muon capture (OMC) to the relevant
intermediate states. The first results from such experiments are expected to
appear soon. As an example we discuss the $\beta\beta$ decays of $^{76}$Ge and
$^{106}$Cd and the corresponding OMC for the $^{76}$Se and $^{106}$Cd nuclei in
the framework of the proton-neutron QRPA with realistic interactions. It is
found that the OMC observables, just like the $2\nu\beta\beta$-decay
amplitudes, strongly depend on the strength of the particle-particle part of
the proton-neutron interaction.
