Insanity is a major gray area when it comes to defining it. Typically we think of someone insane as being someone completely unable or unwilling to grasp cultural norms, causing them to behave in an unusual manner, especially when those people are unable to think of the consequences of their actions. People whose actions harm others are typically consider sociopaths, while people whose actions harm themselves are considered mad.
There are varying levels of insanity, and different kinds, too (although I'm sure you're all aware of that). If insanity is loss of control, then any part of the brain could be affected by this. There are people who only crumble when exposed to something that triggers them. This is extremely common in PTSD. Then there are people who unconsciously participate in 'insane' behavior, which may happen as a coping mechanism or could just be a habit that's followed obsessively.
Depending on what is preventing your brain from functioning normally, you may or may not be aware of your insanity. But if you can still recognize what is considered normal, and still have adequate enough judgment to be able to compare your idea of normality with your own behavior, then you should be able to understand your own level of 'sanity'. Those are really the only tools you need for such a generalized self-diagnosis.
Of course, both of those ingredients are rather loose in their definitions as well. Perceiving what is normal, for instance... Your idea of normal is based entirely off of your context growing up. That could be anything, including isolated cult-like communities.
As far as judgment goes, just look at all the assholes in their giant pickup trucks driving recklessly, then blaming everyone else when they almost get into an accident. Or look at women who are completely unable to accurately determine their own weight in comparison with other people.
In the end these things are based off of culture. The more you can separate yourself from an individual culture and get the big picture of how different cultures are similar and different, the better you can give a singular definition to such a broad term as 'sanity'.