I question my sanity quite a bit, but I tend to use Birds definition of insanity. If one can interact with the generally agreed upon reality, and separate their imagination from it, they are probably insane. Also, the truly insane often do not think that they are insane.
I'm not schizophrenic, but I definitely think I have some weird brain chemistry. I get into weird disassociative moods at times, my vision goes and walls start "breathing", I get weird halluncinations/flashbacks, and things like that. It's all generally very mild, and I can continue to interact with the world, but my "changes of consciousness" didn't start until about 2 years ago, and I believe they were self "inflicted."