There is no "tell tale sign."
It's too different from one person to another. It is possible to become convinced and rightly so by doing one's own research, but it does take time.
There are no simple answers. There is only coming to understand it, and yourself, and long term consistency in the "answers" being there every time you look. At first, you suspect, then you become more and more convinced. Then, at some point, you SAY you are convinced, but may still have doubts. When the doubts finally fade away entirely, after a time, you just KNOW.
It IS possible to learn enough about it, and yourself, that you actually understand it better than some professionals who diagnose it.
But, it's not a quick journey. It took me over ten years.
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I'm not likely to be around much longer. As before when I first signed up here years ago, I'm finding that after a long hiatus, and after only a few days back on here, I'm spending way too much time here again already. So I'm requesting my account be locked, banned or whatever. It's just time. Until then, well, I dunno...