Statements in the form "Everything is ______" tend to be accepted via abduction (SEP). In abduction, you infer that a hypothesis is true because it is the best explanation for a behavior. Abduction, unlike deduction, does not prove the statement, but rather simply infers it.
Statements in that form can also be subjective. It is entirely possible for someone to claim "everything is ______" and simply make themselves incapable of being aware of anything which is not part of that everything. This is often a self fulfilling prophecy. For example, if you believe everything in the world is made of matter (materialism), you are very unlikely to seek out and observe anything which is supernatural, such as a mind (dualism). Your statement can be true about your world until something forces you to be aware of a counterexample.
I do believe small children accomplish this the best. They have a miraculous ability to assume "mom and dad cannot see me while I do this" so completely that they can utterly ignore mom or dad standing right behind them. They are aghast when they find their plans foiled, as though mom and dad somehow ruined their perfect world.
An interesting example of this occurs in the book Permutation City by Greg Egan. In it, they create a simulated world built out of simulated automata. Some of the characters develop a "Garden of Eden" pattern which can provably never evolve naturally. The presence of this pattern proves that you are in a simulation. When they attempt to use this Garden of Eden pattern to disprove the beliefs of the denizens that they are not in a simulation (an "everything is real" argument), well, I wont spoil it, but I will say there are many creative ways to avoid being exposed to such a pattern!