How can we know anything about reality?
We know reality by comparing records where independent observers both looked, and that overlap is the only place a fact about the world can form. A single instrument reporting a number gives a fact about the instrument. On 4 July 2012 two detectors at CERN, built by different teams to different designs, each blind to its own signal region until its method was fixed, announced a new particle at 126.0 and 125.3 billion electron-volts. Their ranges overlapped, and the overlap made the particle a fact about the world. Nobody can step outside to check, because every ruler and balance is built from the world being measured. What survives every comparison anybody can make is all there is, with no further fact behind it.
Also in Chapter 3: Why Would a Universe Need Anybody In It?, Chapter 35: What Is Anything Made Of?, Chapter 11: What Does Everybody Actually End Up Holding?.
This answer comes from OPH, Observer Patch Holography. OPH is a rigorous, consistent and complete description of the unified, self-creating computational Universe we inhabit. Its consistency is machine-checked: the fixed point at its core is constructed explicitly, and a theory with a model contains no contradiction.
To understand OPH and check every step yourself, start with the free book Reverse Engineering Reality. It builds the theory from the ground up, and the OPH Notebook answers your questions while you read.
read the free bookask the notebookor ask sage:read chapter 4: Why Do Two Descriptions Ever Agree?watch a universe make its own particles