My neighbour said he went through some sort of breakdown in his 30's. He was asking people to look after his record player because he had this idea he wanted to go and live in a nearby park. He said the whole experience felt like seeing the light though- I think- for him- he felt the most clarity he had ever felt.
I expect it's different for different people. I've seen people with dementia- some are more aware of their condition than others. I think in some ways- it would be worse to know you were losing your faculties.
I imagine we'd all be considered mentally ill by some folk! Just for seriously considering CTB. I expect there are different degrees of insane.
More's the point though- how do you define sane? Is it the way the majority of people think? Is it what you're told? Ideas on sanity have changed a lot throughout history. Back in the days of assylums, the most benign of things could get you committed (novel reading, laziness, tobacco and masturbation)
A list purportedly documents the myriad reasons or symptoms behind patients' being admitted to an insane asylum in the 19th century.
www.snopes.com
I imagine- if those around you notice you are behaving strangely, maybe something is up. I'm not so sure it's something we ourselves might be aware of.