Well yeah -- meta-cognition is a boon and curse at the same time. On one hand it allows us to create civilizations and appreciate creation with added symbolic dimension, on the other hand it creates an illusion of separation with all psychological problems and their effects that go with it. As of now (almost 100K years and counting), we struggle to use it properly. But proper handling of meta-cognition is an evolutionary given (or we go extinct).

https://senderspike.medium.com/the-original-death-cult-166c95898a9f

As for your question -- you would know it by verifying that all attributes, properties, phenomena, etc. are what is observed/known. Thus, "I" is attributeless and unsubstantial, which makes all "I"s spatiotemporally uniform to the point of absolute identity. Or in other words, each seemingly separate "I" is identical with any other "I", which makes them all indistinguishable and literally one.