Observation does not decide, it gives us the facts. It is analysis (Viveka) which takes us to truth. That is meditation (Dhyana), contemplation. Knowing 'what exists' is not difficult. What did we start with at the time of Big bang? Physical Energy. That is what exists. It is about the properties of energy that we do not exactly know. What gives rise to this energy? What is its relation with space and time? Etc.
What is analysis that it can be said with confidence that it is more like reality than that which we experience directly with our senses? True, we can suffer under an illusion, but what illusion is ever dispelled that is not aided by further sensory awareness?
Simultaneously, what sensory awareness can we not deconstruct and reveal its arbitrariness? What are colors? Are they properties of reality or something the brain has co-created with the reality that appears to reflect colors? What are thoughts but the brain activities that were co-created with the reality that appears to conform, or not, to those thoughts? Is red really what a certain range of electromagnetic radiation is? Are the equations used to describe the radiation showing us the nature of red?
Every perception is true and false. Every thought based on perception is also true and false. Intuitions which seek to find the patterns in these things are also true and false.
Perhaps Brahman is the intuition that this pattern is complete across all cognitive experience and that we can, in a cautionary moment of additional cognition come to the idea that Brahman transcends our cognition of reality.
This can neither be proven nor disproven. Like color we co-create Brahman with the reality we perceive to "embody" Brahman. In this sense it makes as much sense to say we are Brahman.
In the realm of common sense based on sensory experience this sort of thinking dissolves rationality making distinctions useless and soon becomes tiresome. In the realm of spiritual awareness which is based on intuition this sort of thinking integrates rationality transcending distinctions and brings a sense of peaceful wholeness.