Several signs that your nearing the break of insanity would be hearing voices, extreme paranoia, feeling like the world is conspiring against you and unreasonable shouting when talking to yourself
You don't. If you knew you were insane, you'd be sane. Strangely. There's nothing wrong with being strange. It's actually normal to be strange. So go insane.
If i am going insane i will do things which a normal person can not do. This might be unnoticeable in the beginning but when people close to me tell me about my actions which were not part of my personal nature then it will be a sign for me.
If you see things that are not there, hear voices no one else hears, or have wild mood swings, then you need to see a psychologist for a medical evaluation.
