  Electron spin oriented along an external magnetic field is subject to
longitudinal spin relaxation with characteristic time $T_1$. The corresponding
decay is nonoscillating, so one cannot readily ascribe $T_1$ to a certain $g$
factor. This becomes a problem when several electronic states with different
$g$ factors are present in the system, e.g. electrons and holes. We solve this
problem by optically pumping spin polarization and then selectively
depolarizing it using a radio frequency (rf) field. By modulating the rf field
one can observe the retarded modulation of spin polarization which depends on
the relation between the modulation period and $T_1$. Using this selective spin
inertia method, we unveil the strong anisotropy of $T_1$ for rare-earth
Ce$^{3+}$ ions in a YAG crystal at low temperatures and low magnetic fields. We
also show that the spread of Larmor frequencies within the electron ensemble in
this system is not static, but results from the fluctuations of internal
magnetic fields on a timescale much shorter than $T_1$.
