  We propose the Casimir effect as a general method to observe Lifshitz
transitions in electron systems. The concept is demonstrated with a planar
spin-orbit coupled semiconductor in a magnetic field. We calculate the Casimir
force between two such semiconductors and between the semiconductor and a metal
as a function of the Zeeman splitting in the semiconductor. The Zeeman field
causes a Fermi pocket in the semiconductor to form or collapse by tuning the
system through a topological Lifshitz transition. We find that the Casimir
force experiences a kink at the transition point and noticeably different
behaviors on either side of the transition. The simplest experimental
realization of the proposed effect would involve a metal-coated sphere
suspended from a micro-cantilever above a thin layer of InSb (or another
semiconductor with large $g$-factor). Numerical estimates are provided and
indicate that the effect is well within experimental reach.
