Well, it’s all mental. In our heads, what we do and how and why we act is normal. So while a person’s mind is developing and changing as they turn insane, they wouldn’t notice – as to them, their new attitude makes perfect sense, as they’ve experienced it develop, and possibly haven’t noticed a difference. Also, should someone suggest you were insane, you’d deny it, wouldn’t you? We’re bound to side with ourselves, and besides, an insane person wouldn’t tell the difference as it’s what they know and believe to be normal. A professional psychiatrist would give the best diagnosis for insanity.