  For materials near the phase boundary between weak and strong topological
insulators (TIs), their band topology depends on the band alignment, with the
inverted (normal) band corresponding to the strong (weak) TI phase. Here,
taking the anisotropic transition-metal pentatelluride ZrTe$_5$ as an example,
we show that the band inversion manifests itself as a second extremum (band
gap) in the layer stacking direction, which can be probed experimentally via
magneto-infrared spectroscopy. Specifically, we find that the band anisotropy
of ZrTe$_5$ features a slow dispersion in the layer stacking direction, along
with an additional set of optical transitions from a band gap away from the
Brillouin zone center. Our work identifies ZrTe5 as a strong TI at liquid
helium temperature and provides a new perspective in determining band inversion
in layered topological materials.
