  In this paper, we investigate signatures of topological phase transitions in
interacting systems. We show that the key signature is the existence of a
topologically protected level crossing, which is robust and sharply defines the
topological transition, even in finite-size systems. Spatial symmetries are
argued to play a fundamental role in the selection of the boundary conditions
to be used to locate topological transitions in finite systems. We discuss the
theoretical implications of this result, and utilize exact diagonalization to
demonstrate its manifestations in the Haldane-Fermi-Hubbard model. Our findings
provide an efficient way to detect topological transitions in experiments and
in numerical calculations that cannot access the ground-state wave function.
