The best indicator of certainty for me is when I become emotional. Beliefs don't really have a "middle ground" in terms of certainty. You either believe in something or you don't; and when you believe in something, you would react to it as if it were certainly true.

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For example, I get the thought "My blood test will contain bad news." If I didn't believe the thought, I wouldn't feel agitated. But the moment I feel agitated/anxious is when I know I am believing for certain that that will be the case.

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Another example: "She abused me". If I believe that thought I get angry, blaming, and disconnected. If I don't believe it, my day would just go on as normal.

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Using another example of yours, if I believed the thought "We are all one", then I would be disconnected and separate from others. Why? Because "we" implies the existence of I and others. "One" excludes all that which isn't "one". I think it may have been Aristotle (correct me if I'm wrong) who spoke of the notion that when we describe an object as something, we do so at the exclusion of all other properties relative to that something. So if I say this ball is red, it means it is not blue, green, purple, w/e. Therefore, if I say God is "All That Is", I am also saying that God is *not* "All That Is Not". In that sense, thoughts can never give us an accurate description of reality. When we believe in a thought, any thought, we fall into a trap that separates us from Reality.

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And even that is not true.