Can we know if "all" physical laws are discovered? Before Einstein, scientists believed that all physical laws to be discovered were already discovered. Of course, that was proven wrong.
If we somehow manage to come up with the Theory Of Everything that even explains dark matter and dark energy, can we know if the TOE is all there is?
If we cannot know that there is nothing beyond the TOE, it might be presumptuous to call it TOE, like scientists did before Einstein. But since scientific method is inductive, I think it is impossible to know for certainty. (And hence the name, theory.)
 A: 
can we know if the TOE is all there is?

One of the main principles of science is falsifiability. All scientific theories are falsifiable. A theory may explain all we currently know but we always have the hope and expectation that our knowledge may expand. We must not know if things we don't yet know will also conform to current theories. That is an essential principle.

And hence the name, theory

Many people misunderstand this word and the way it is used in science. Some lay people might say "evolution is only a theory" or "gravity is only a theory" in a dismissive way as if to suggest that theories are no more than uneducated guesses.
The way I understand the progression of ideas, along a path of increasing certainty, is as follows


*

*Conjecture: An educated guess of the form "perhaps X occurs because of Y" where X and Y might be complicated.

*Hypothesis: A more fully worked-out idea, often one that can be expressed mathematically, is capable of making new predictions about the universe and which is therefore testable.

*Theory: A hypothesis which has been subjected to very extensive testing to try and find circumstances in which it can be falsified.
So, in science, a theory is a pretty substantial thing.
A: 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.
A: Here's an answer from a slightly different, more philosophical perspective.
Physical laws are not "laws" that describe and embody some absolute truth.  The entire endeavor of physics is to find mathematical descriptions that match our observation of how reality behaves, and allow us to make predictions about future observations.  
A "law" is just the best current description, and is always subject to revision, adjustment, or even being completely discarded if some future observation doesn't match its predictions.  In fact the most important advances in physics usually come from such discrepancies.  As various people have said, physics revolutions usually start with some one saying "Hmmm... that's unexpected...".  For example, the orbit of Mercury not matching Newtonian predictions.
So the answer has to be, a priori, that we cannot ever discover ALL physical laws.  Even if we reach a point where we're not discovering ANY discrepancies, there will always be realms of precision we cannot measure, or avenues of research we haven't thought of. Heisenberg and Godel conspire to ensure that we can never know everything.
A: 
If we somehow manage to come up with the TOE that even explains dark matter and dark energy, can we know if the TOE is all there is?

Take the current favorite candidate for a theory of everything, a string theory to be decided in some future while we are still alive. It will model all of particle physics,  quantize gravity all in one mathematical structure. 
If it succeeds it will be the current TOE, and since it has all these dimensions there is a probability that new experiments, not thinkable presently, will be proposed from this TOE to try and falsify it. History of of physics tells us that the probability is high that at some values of coordinates, energies, ..., it may not hold up, showing that it is not really a theory of everything, but the last mathematical model in a long row of models modeling nature.
