Thermodynamics, electromagnetism, and classical mechanics were the "TOE" of physics at the time of Lord Kelvin. They remained a "TOE" until new phenomena were discovered, which could not be explained by their theories, such as the photoelectric effect, the energy quantization of photons, the atomic spectra, and many others.
In the same way, a TOE which would unify quantum mechanics and general relativity, describe dark matter etc, will remain a TOE until new phenomena will be discovered, which will contradict the TOE, or simply cannot be described in its framework.
One can say that a TOE is a TOE until some unexplained experimental phenomena falsify the TOE.
EDIT: There is also one other consideration. The term "theory of everything" in its current meaning is misleading, because it is not a theory of "everything" in the strict sense. There are physical phenomena that are not addressed by the TOE. For example, chaos dynamics, non-equilibrium thermodynamics, solid state physics, biology and many others. Proponents of a TOE argue that such theory would explain in principle all known phenomena, including chemistry, biology, etc. This is logically correct, but practically useless. Of course chemistry and biology are "in principle" already explained by the physical laws currently known. But practically, nobody has been able to reduce Darwin evolutionary theory to quantum mechanics. There are physical phenomena that are simply beyond the scope of a TOE.